I owned some PowerBlocks. They were great space savers. Went up to 125 lbs and sat in a 2' x 2' area. They take a little bit of getting used to though. Your hand is down inside them and its sorta like having some robotic looking attachment on your hand. Now - since you are in your homegym it isnt like there are people staring at you or something. So you get used to them. I dont recall any downside to them other than that. I didnt ever throw them around so Im not sure how durable they are for that. As I recall, the ends of them are made of plastic, so I would think they would crack if you throw them down too often.
I have seen the bowflex dumbells but never used them.