Powerblocks look dumb. I also am a little unsure about the way you stick your hand inside of them to hold them... maybe it's just me, but if the price was in the same neighborhood, I would stick with what I know.

Also, while being able to add extensions on the Powerblocks seems like a great idea, honestly, I don't think most people would ever want/need that. I guess it depends on what you're using them for. If you're building a garage gym so you never have to go to the gym again, wouldn't you have a bench press bar that would allow you to do any lifts you would need heavy dumbbells for (bench press, overhead press)? If you just want to have some workout stuff around the house for the Saturday morning when you just want to pump out a few sets, would you really need anything heavier?