I think powerblocks are generally quick to change and gets quicker as you get used to them. You have 2 inside cylinders that give you some more granularity but mostly you'll be using pins to change in/out. I've seen too many Bowflex SelectTech's in stores that were broken to go that route plus the SelectTech's are generally very long. The Powerblocks are shorter, impossible to break but they are still bulkier than normal dumbbells. Also, dumbbells don't generally allow for granular transitions as they start with 5 lb jumps after 15 lbs. Your hands are inside a cage of sorts which seems odd at first but works well.
I'd definitely look for a store that sells them to get some hands on look at them. I think the place in my area was called Fitness Resource but any serious fitness store should have them.
You can't out-exercise poor eating habits.