I know they're not very practical, and probably not very accurate, but I'm thinking about getting one just for a "fun gun". So...

How reliable are they? I'm not looking for "bet your life on it" reliability, just want it to consistently go bang enough to have a good time with it.

What should I watch for, and watch out for, when shopping for one?

The actual M11/9 is a $2500-2700 gun...

Cheapest MG you can find, and there are about as many widgets & whatzits for them as there are for ARs,... Including a process to make them use the AR recoil system (CAR stock & buffer), lowering ROF (a GOOD thing on this gun) and increasing accuracy...

It's really not worth it in semiauto, though...

Quoted: The actual M11/9 is a $2500-2700 gun... Cheapest MG you can find, and there are about as many widgets & whatzits for them as there are for ARs,... Including a process to make them use the AR recoil system (CAR stock & buffer), lowering ROF (a GOOD thing on this gun) and increasing accuracy... It's really not worth it in semiauto, though...

I shot mine this past weekend.  I shot about 20 or so while on Semi.  The rest of the 850+ rounds (24 (magazines) times 32 (magazine capacity) plus 100 (reloads)) were done on Rock & Roll.  Including a few Mag Dumps, too.

While I was on Semi I out shot somebody in the next lane that was shooting a .40 pistol.  I almost out shot her while on Auto as well.  Yeah, the other shooter was a gal.  She said she was in the Air Force.  I asked her, “You did have some weapons training, didn’t you?”  She said she was in the Air Force for 10 years.  So I asked her if she had any weapons training.  She was bad!  I was hitting the target at 25 yards and she was having trouble hitting the same target at under 10 yards.  Except for Mag Dumps I was out shooting her very easily.
She was so bad it was scary.

But to get back to how well my Mac can shoot on a Good Day, two years ago I shot two magazines (50 Wolf) through my Mac on Straight Semi.  The groups I had were better than I had ever done with my PPK.  The Mac can be reasonably accurate.

I've got a MAC10 and know a couple of guys with MAC11s. To sum it up, MACs are fun little buzz guns. Accuracy is fairly good, reliability depends a lot on your magazines, and the fun factor is pegging the meter. Plan on spending lots of $$$ on 9mm and having sore thumbs the first couple times out.

The only real thing to watch out for is LEATHERWOOD Texas MACs. They were poorly made with spot welding. The SWD Texas MACs are well built little guns. Powder Springs and RPD MACs have an excellent reputation but also command a bit higher price.

... I bought like-new Ingram MAC-10, chambered in 9mm w/5 mags last year

... Paid $1150

If you're talking about the simiauto, it sucks.  Trigger pull hurts after a few rounds, and not reliable .  If your get the hi cap mags you need a different mag release.

they go bbbbbrrrrrdaaaaaaaaaaaaaaaaaaaaaaaaaaaaattt

Simiauto? Is that a gun from The Planet of the Apes?

Yes, I'm talking about the semiauto for around $200 or $250.

A member here turned me on to these guys a while back when I asked the same thing. Got one and I dig it.

www.rpbusa.com/ffl.html

I had a semiauto Cobray M11/9 for while, it was actually fairly fun I thought.  Mine was fairly reliable.  It would be alot more fun if it were legal to put a buttstock and K-grip on em without the $200 SBR tax BS though....  Anyway, they're cheap and fun, and actually aren't that inaccurate either...  I want one of the new sidecockers that takes Sten mags that a couple companies are selling...

I have a Cobray PM11/9mm.  It's a "looker" in that everyone at the range thinks it's a machinegun, but the trigger slap is horrible, and after about 10 rounds, it's no longer fun to shoot.  Mine does have a little "pad" on the trigger that helps a bit.  Coincidentally, it's for sale - well, if you're local in AZ!  

I think they are fun, mine is 100% reliable with fmj.

I have an SWD MAC11 and purchased a long barrel upper for it. I like it a lot better and it's more accurate. I picked up the gun for $1500, NIB a couple of years ago. One of the last ones made and never fired, up until I bought it.
community.webshots.com/scripts/editPhotos.fcgi?action=showMyPhoto&albumID=97564046&photoID=176823824&security=nugEtc

my friend has one, full auto.
it had the selector switch break.
it was replaced and was good for awhile.

it started not going full auto on him, he gave it another couple of shots, and the whole back of the receiver failed and fell off onto the ground.
the weld job looks nasty, but it's working again.

it's an entry level machine gun, and lots of fun when it works.
even though it's cheap, the value keeps going up.

I have an 11/9 and it runs good. Get the South African mags for 20 bucks and save the plastic
orginal mag for when you get rid of it. Mine is select fire and has the sliding wire stock and I have a front grip that screws on the threaded barrel.

Quoted: If you're talking about the simiauto, it sucks.  Trigger pull hurts after a few rounds, and not reliable .  If your get the hi cap mags you need a different mag release.

trigger slap from hell i tell ya...