Disclaimer: I can add up the number of minutes I've personally spent with grappling dummies without taking off my shoes. (And by "grappling dummies" I don't mean "judoka who've had too many concussions".)
From a RGA London student on /r/bjj, a disclaimer:
If you're fairly new to grappling I would stress that it IN NO WAY is a substitute for training with real partners and that you'll still need to do that all the time.
...and some answers to specific questions:
Q: Can you put them into different positions and expect them to be able to maintain those positions given no outside interference?
A: It depends:
You can probably imagine basically which moves it's possible to do with a dummy, but here's an extended take. Standing guard passes work really well, as you can basically bend the limbs out to mimic somebody spider-guarding you - although obviously you won't get the same pressure. Moves from mount and side mount are great. Leglocks (I've been working on some stuff from Reilly Bodycomb which are against BJJ rules) work great. Submissions from guard are more tricky, because it's difficult to get the right pressure from the dummy - but I've been practicing some rubber guard stuff with reasonable success. Sweeps from guard are tough, because you can't really get the dummy to posture up on its knees properly without tying its legs together. Guard breaks aren't that useful, although you can simulate a closed guard/halfguard by tying its legs together. Escapes from mount and sidemount aren't the best because you aren't getting proper pressure from your opponent. Moves that require the dummy to be on hands and knees (stuff from sprawl control, say) require you to tie the dummy's legs together. Anything with you on the ground and the dummy standing clearly won't work. A general rule of thumb is, if you need to 'feel' an opponent's pressure or balance to make a move work, you aren't going to be able to drill it properly with the dummy. The website claims that you can practice throws on it, but that seems ridiculous. That still gives you a lot to work on, though.
I am surprised and a bit dubious that he doesn't see the benefit of practicing throws on an object that doesn't whine when you makikomi (land on it). Perhaps that's more useful for people who already understand the mechanics and pressures of a throw and just need to ingrain the total commitment necessary to throw hard.
Q: When you attack a joint, how much resistance is there?
A: It's not meant to mimic a human's resistance, it's meant to mimic a body's mass. So, no, it won't try to stop your armlock and then have the same amount of tension at the elbow joint. From a related /r/bjj post, this time regarding a homemade dummy:
[He] comfortably bends his joints at all the same angles as we do.
If you try and force it at points where the natural tap "zone" would be you meet resistance. The wiring is electrical service entrance wiring as well so provides stiffer resistance when you go against the natural curvature of Bubba the majority of the time.
Q: How lifelike are the joints?
A: If you take a look at the pictures of this homemade dummy, obviously the answer is "not terribly". The answer above suggests that commercial dummies are better, but I wouldn't expect much. These are more "punching bags with limbs" than crash-test dummies. Again, we're replicating the mass of the body, not its actual physiology.
But I'm not sure that's the point. As the homemade-dummy-maker explains, the purpose of the dummy is to drill copiously, or drill when no partners are around, not to perfectly mimic the feel of a real human:
How much use this dummy is going to be to you is down to two things: how convenient it is, and how dedicated you are. I've got a girlfriend who lets me leave it in the living room for days on end and at worst put it in a cupboard, and as such it's always there - if I had to go train in the garage with it, or wait until I was alone in the house, it wouldn't be as useful. As it is, it's like having a training partner who never demands a go, or says he's tired, or doesn't fancy training. If you want to do a move 100 times you can. If you're reading an instructional at midnight and need someone to try the move on, it's there. If you've got ten minutes before breakfast, you can bang out some reps of that move you're going to try in the evening. I can't emphasise enough how useful this is.
But that wouldn't work without discipline. I enjoy jits, but more than anything I enjoy feeling like I'm getting better at it, and going from class to class without a structured way to improve was getting to me. Training with the dummy is boring, but if you've got the discipline to do those 100 reps, it will get you results. Look at it this way: if you're the sort of guy that ends up chatting to your partner during the drilling bit of class, you might not get much out of it. I'm a big believer in Eddie Bravo's idea that doing a move should be as instinctive as tying your shoes, and if you do enough reps, it will be.