OK, so I have some scope body bubble level attachments that I ordered with some SWFA scopes. I have mounted them and remembered to use them, but only occasionally.  I figure that is more training than anything else.

I also have one built into the bottom of an Aadmount and others mounts incorporate them.  I have also seen some picc mount levels.

Obviously these are aimed at precision or long distance shooters where a canted reticle will hurt more than at short distances.

What are people's opinions on using these?

You pretty much answered your own question. Really aren't terribly useful at shorter-mid range and they also don't do anything for you if the level isn't mounted perfectly with the cross hairs any way.

Consistency is the key to accuracy.  

This is just one more factor.

I've got some bubble tubes that I use to get the scope level when mounting it in the rings, but not attached to my scope/rifle to reference while shooting.

It lets you now if you're shooting from uneven ground while in the prone. It may not matter to dirt clod shooters or people that don't care, but it matters to at least me when I'm slow shooting for groups (testing loads or ammo from manufacturers). How else will one know if you bumped your rifle to an uneven spot and made a potential 1 MOA or smaller ten shot string for consistency worse than it should have been?

You have to be shooting looooong distance for a degree or two of cant to have any significant effect.  For and AR I say meh.  For a precision rifle shooting 1k?  Perhaps.

I bought a 30 dollar one that has a rail mount, for my Remington 700.  It's good for seeing how level your bipod set up is, and if the bench is slated.  I've seen more expensive ones for way more - not sure if it's worth that, but for 30 bucks it severs a role.  Plus looks cool, like you're a super shooter, instead of a spray and pry type (like most of us!).

Actually if you are trying to shoot sub-MOA groups at 100m a cant will hurt your chances more than you think.  Next time you are at the range shooting match ammo for as tight of a group as possible cant your rifle some and if you are doing all your part you will see your groups will have little shifts impacting your overall group size.

At 100 yards, even canted one degree would & will be, significant.

And, NO.  According to an expert "Bryan Litz confirms Tubb’s observation. Bryan tells us that, as a general rule of thumb (for common cartridges), a 1° cant will produce five (5) inches of lateral displacement at 1000 yards."

So we are talking about 1/2" or less @ 100 yards for 1 degree of cant on a calm day.  Significant?  Meh!  A bench gun shooting handloads?  Sure, but not for your typical off the shelf AR shooting steel case FMJ's through a BCA barrel

For plinking it is not a big deal you are correct. If you are trying to group with match ammo or had loads trying your hardest to print up the tightest group possible it is a factor.  That basically could be the difference between a 1.5MOA gun and 1MOA or sub-MOA.  You also assume you will be only 1° and not 2 or 3° canted.

And NO back to you as well. You even proved my point that 1/2 is significant enough at 100 to disturb consistency when doing load development with ether handloads or factory loads.

And ? never sad that ? was shooting typical steel case through a BCA, that was all projection on your part and has nothing to do with how ? shoot.

You do you.

He said 1 degree.  I simply responded to that.

I only shoot handloads.  I was simply speaking about the typical Joe out there.

So do I. Almost entirely from ether an AR15, AR10, or an R700 are handloads. Most of my handloads are for the AR15. But for any of them I need to make sure that the earth I am shooting on is flat or level or I'm wasting my time, money, and resources.

But I get it. You rely on the eyeball to know if you're leveled or not. That's cool an all but for others that find the value in it enough to spend the money for it that is not taking money from ammo funds, "typical shooters" or not is irrelevant; a good piece of it is still a good piece of kit and they not you, should decide for themselves if it is right for them.

Now speaking for myself here, all of my precision, zeroing, or load tests, are done in the forests or desert. On mostly uneven ground. It's nice to have a level that tells me if I need to extend a bipod leg a notch down or two before getting behind the trigger.

Mine are barrel-mounted out at the front sight.

If nothing else, it reminds me to focus on the front sight.

If you live in a hilly area where there's not much of a level reference other than your natural balance, yes. If you have an unrefined position or a weird shoulder shape that twists the rifle during recoil, canting the rifle relative to sight and using a sight mounted level may be effective.

5* or less isn't significant in practice IMO.

I have the US Optics one. I like it. I have no problem with them and do not see them as a gimmick (though I'd question why it's on something like a 7.5" pistol build).

Agree, but . . . Those of us who have these levels on our scopes or rails are likely the very ones who do shoot match ammo for tiny groups or long range precision shooting.  For us, it does matter, even at 100 yards.  A 1 degree cant is a half inch lateral shift at 100 yards.   And, without the level the cant may be 3 or 4 degrees.  When you are shooting sub 1/2 MOA rifles and ammo capable of close to 1/4 MOA with ideal technique, it matters.  Even 1 degree is a blown group.  

And, stretch it out to longer yardage and cant matters even when hunting or shooting steel.  While discussing cant, don't forget its not just lateral cant that matters.  Shooting even slightly uphill or downhill affects bullet flight time and effectively changes distance.  That alters your hold over at longer ranges.  My VX6-HD scope has a built in  cant detector visible through the reticle.  It's there for a reason.

If the shooter has a rack grade barrel shooting bulk 55 grain ammo, don't bother with a level, true.  But, these devices do have their merit on more accurate rifles shooting better ammo.