I got two sets of these yesterday: warlord because I play a warlord, and warlock because one of the two players in Adrian's game plays a warlock and I wanted to see how another player handled them. Then I kicked myself because as a resourceful warlord, all my shit was in Martial Power, and those wont be out for awhile. I kicked myself again later when I realized that Corinne was a dark-pact warlock, and I just should have broken down and got a paladin set for Jennifer because she's the only one using stuff from Player's Handbook.
The power cards are as you would expect: they take the entire power block from whatever set you have and format it to fit a card. They are more durable than the paper you might print a sheet on from the Character Builder, but obviously dont have all the values entered in ahead of time. You could probably do this, but then it would mess them up right-quick.
They are sorted alphabetically by level, so its easy to skim through them to find what you want, and there are quite a few blanks in the back that you could use to enter in values for powers you have, create your own, or to just jot down the ones that arent on cards yet.
I think mostly these will be useful for players that dont have the Character Builder, but want something that looks nice. Its generally not hard to figure out what your attack bonus is, and they make an easy way to track power usage by flipping them or something so that players dont "forget" what they've used. Doubly important between games if your group likes to take an extended rest whenever the session winds down (*cough cough* Corinne and Jennifer *cough cough*).
So to surmise, they're durable, look nice, and are easy to navigate. You can also opt to not print out the card section of your sheet and save up on ink.
The downside is that if you've rememberd all your stats or pick anything out of Player's Handbook you're screwed until the Power-Souce-Name Power set comes out...unless you write on the blank ones.