I got one to use with Chris Carmichael's "5 Essentials" plan. I got an 8 lb. one, which for most stuff is perfect for me but is too heavy for the one handed-extended arm exercises. I use some old ankle weights in my hand for those.
I went with a cheap Amazon deal for a Valero brand "weighted fitness ball". It's fine...supposed to be able to use a ball inflator to soften it up, but that hasn't really worked, so it stays hard and a touch flattened. Not a big deal for what I do, I'd use a rubberized one for "toss it around and throw at the wall" type exercises.
Good timing on the post -- I'm about to restart the "home gym" aspects of the program to get my upper body strength back up.