I have one but I never used it. The amount of poses you could make with it is very limited. The joints could only stay on it's own up until it reaches a 90 degree angle. It's always better to have a friend pose for you.
The main thing an artists mannequin will help you do is draw poses that look like an artists mannequin. If you rely on it for reference then your figures will literally look wooden. They're much more useful for artists who already understand anatomy and use the mannequin as a quick base reference, but who understand what to change in order to make the image look natural.
I have one, and've only ever used it as a shelf ornament.
I actually won mine because I bought so many art supplies at the local art store. It is neat, but I don't feel it ever really helped me learn poses or anatomy. It worked well for simple sketch poses, but I feel I had to often use my imagination more than I had to use my eyes. I mainly used it for simplistic poses for my drawing classes where we had to sketch out stick figures doing different things. The art stores where I am sells them from $5-20 dollars.
There is a really good website I found back when I used to do figure drawing that shows you poses- I believe it is called posemaniacs. That website really helped me out, perhaps it is worth a look if you are into poses/ anatomy.
I've never felt inclined to buy one. I've found that in most cases if I need to know what something looks like, I can just walk to a mirror and observe how I look in that position as for one or two joints it's not going to vary too much.
If you're doing things properly and drawing out a skeleton before you draw the rest of the body, you should be able to change it hundreds of times if you need to with minimal wasted effort.
Come to think of it, I don't think the school I took drawing and painting at used them at all for classes. I guess the principles of drawing are the principles if you can draw flowers then you should be able to figure out how to draw people given enough observation.
One thought though is to watch video of the joints moving. I'm sure there are plenty of videos online and that's going to be more useful, IMHO, as it shows the range of motion that the joint goes through and often times the reaction in the rest of the body.
Not really. I bought one for pose references years ago (I don't even know why you'd use one to actually study anatomy), but it seems to have... springs in its joints or something, so if I wanted any of its arms or legs to be in certain positions, I had to bust out the scotch tape to hold them there. There's also the escher girls example kafine linked to, but I think if you have a decent grasp of the human body's range of motion, that's not too much of a problem.
If you're looking for something to pose around for references, the best thing I've tried is a 3D modeling program. Which still isn't the best for anatomy, of course, but for just poses it's a pretty alright idea. Still requires the grasp on range of motion, though.