These are pretty affordable given the cost of most other bullpups so I want to pick one up.  How are these as far as reliability and trigger (relative to other bullpups)?  Where on the bullpup scale would you rate the RDB overall?

It’s the best bullpup design ever in my opinion. Better than Tavor, AUG et al. The trigger is good for a bullpup because of its clever geometry. I just wish the build quality was better - using screws to hold the revived together is cheesy, and the mag catch is a poor design. With this said, mine has been 100% reliable.

Oof. That's a big call.

By the way, do you reckon there's a way to mod the receiver to something better than screws?

My cheating skank ex was a leftie so I bought a Beretta ARX and Kel-Tec RDB for her to use.  Ditched her, kept the guns!
Anyways, the RDB has been 100% reliable and pretty accurate as well.  Of course, it is ambidextrous to where even the cocking handle can be swapped around so if you’re a lefty, this one is probably the bullpup you want to get.  I think the magazine release is a little crude in it’s design but it works so can I really complain?  The trigger is pretty good by bullpup standards although the trigger return spring being exposed may or may not be an issue.  To date it hasn’t been a problem but it’s just something you notice.  
Also, QD sling cups I think would have been a better choice, just add a pair of HK snap hooks to your favorite sling and you’ll be good to go.  Yes, there’s a lot of screws that hold it all together but in all honesty, I don’t really have a problem with it. I just made sure to pull out each screw, wipe them down with alcohol and reinstall them with some blue loc-tite just as a little extra insurance.  
All-in-all, I think Kel-Tec gets a somewhat justified rap of being innovative but not quite as well executed designs as they could be.  Sure, thing like the trigger return spring, magazine release and the copious amount of screws to hold it all together are legitimate points.  However, what can I say, it works, it works well and is at a price point that is appealing.  
If this is your first bullpup and you want to try one to see if you like the concept, this will get you in the game at an affordable (in a Bidenflation world I guess that’s all relative) price.  It’s not the Hellion that is going for over MSRP these days, nor is it the crappy but cheap K-Var AKU-94 bullpup conversion for an AK that I first tried back in the late 90’s that put a bad taste for bullpups in my mouth for years.  
Would I buy one again if I hadn’t bought this one for the skank?  Yes.  It doesn’t have the military pedigree of my other bullpups like my Steyr STG-77 or Tavor but it gets the job done and at a lower price point.

With the adjustable gas and downward ejection port it suppresses better than any other bullpup (or AR for that matter) IMO.

As for reliability, I've had no issues other than when I first got it and kept screwing with the gas adjustment. Been great ever since and would have no issue using in for home defense.

As a go to war rifle, I'd grab something else. The main reason being is that it can be a pia and time consuming to clear a double feed.

I like the design of the RDB, but hate how unintuitive clearing issues can be. Where clearing an AUG is usually as simple as pulling the charging handle and tilting it to the right. Most failures I've seen have needed a breakdown of the RDB.

Honestly where the RDB excels is running suppressed, when you get it dialed in there's very little recoil and it really fun to shoot.

I have an Aug, Tavor 7, and an RDB. The RDB is easily the best suppressor host out of these, and is the best suppressor host out of all my guns.  I am a lefty so the ambi features are really important. I can’t just hand my Aug or Tavor to my righty friends to shoot without going through the conversion process. Downward ejection is cool, but I often wear shirts with chest pockets that are on the left, and hot brass routinely finds its way into these pockets. Damn pockets should be on the right side.

I like mine quite a bit.

Better weight, ergos, and trigger than an AUG for sure.

It's a tie between the FS2000 and RDB for me on which bullpup I like the best.

For going to "war"... I'll take an AR.

I have an RFB,MDRx, and an RDB (soon a Hellion).  Love my RDB and it if that was all I had access to,Id feel fine "going to war" with it (I like my RFB better for that).

Having handled a Hellion and MDRx in 556.. The RDB is (2lbs) lighter than the MDRX, Has more features and a better trigger than the Hellion (No bolt hold open on it).
A am jealous of the Cold Hammer forged Barrel in the Hellion and its something I  wish was in the RDB.

Good write-up.  FWIW, the MDRx is a large chassis gun, that 5 56 fits into too, but really, it shines the most in 6.5 CM.  A .308 based rifle is always going to be overweight in 5.56.

As to the CHF barrel, unless you are running ful auto and many thousands of rounds, that's a lot more sylibles than substinance.  The bullet will be coming out just as fast and just as accurate either way.

I have two and they're my favorite rifle to shoot. Good trigger and easily my favorite suppressor hosts. Both of mine have been totally reliable, though one had to go back to the factory out of the box for a hammer that should have never passed QC.

The only two criticisms I have is they're accuracy is only okay, and they, feel if not cheap, not as rugged as they should. Wish they'd work with someone (Knight's is basically down the street) to make a "pro" version with AUG level bullet proof construction (a CHF barrel would be nice too).

That's exactly what I did. An 20" RDB Hunter was my bullpup gateway drug.

Now I have two RDBs (added an RDB Survival), an MDRX, an X95 and an AUG.

I like them all for different reasons. Knowing what I know now though, if I was only going to own one, I'd go with the X95. However of the five bullpups, the handy little RDB Survival is my favorite to shoot.

The RDB is a great rifle for those on a budget, if you'll willing to accept its limitations.

First one I had was 100% reliable. I was not a fan of the mag release, and I did shoot some screws out. Sold it and played with the more premium pups. Aug, MSAR, Tavor, and KM17. I went back to the Keltec. Newer gen has a stiffer mag release (good) and I removed and I used blue Loctite on the screws so I don't lose any. Second one has been 100% as well. It performs much better suppressed than any of the other bullpup hosts I've used. Downward ejection means no gas i the face, and the highly adjustable gas system makes tuning the gun to a suppressor easy. Trigger is perhaps the best on the market, and it's stock. I prefer the light weigh slim profile of the gun vs others. Decent aftermarket. Shoots left or right handed with no special considerations...aside from charging handle if you're a full time lefty. Really my only complaint is I'd like the mag release to be more robust and, if I'm going to ask for perfection, forward of the trigger of course.

Since going back to a Keltec:
VHS-2: Never owned. Handled and fired. LOP is just longer than I prefer.
MDR: Still haven't gotten my hands on one, so I can't say for sure the RDB is 100% my preferred bullpup.

The rifle is really clever, and interesting,  and a nice shooter. It has too many screws (that love to fall out), and shooting with the gas adjusted wrong will cause hellacious malfunctions. It suppresses well.

It's a neat gun. Not really 'duty' grade.

Mine is older than most..Still havent had a screw fall out..

When I got mine (spring 2020 and spring 2021), I went around the entire guns with a Fat Wrench (torque wrench) and checked the torque on all the external screws. None were loose on either gun.

I swapped the top rails on my two guns and actually had to drill out and replace about half the screws because they were so tight I stripped them trying to remove them. These had Locktite from the factory on both my guns BTW.

The receiver is a piece of pressed  and welded steel  there aren't screws holding it together
It's in the dude's left hand in the picture. The buttplate and the cheeck rest screw into it

@Manticore_Arms

Lucky Irishman Tactical on the forend. Aluminum. Not free floating. Bit longer that Keltec’s defender option, but also flexes a good deal forward of the gas block.

Hand protector is 3D printed to work with the Lucky Irish handguard. Secured with an mlok nut and snaps into the bottom of the grip.

Anyone know where to get this trigger guard? Looks awesome

FOUND THIS ON REDDIT since I was looking for the same thing

3D Printed. Requires the Lucky Irish handguard. Thingverse link: https://www.thingiverse.com/thing:5308919