Ive actually played a bit against Half bloods. The things that I don't like are: They can be very random(good and bad) but it can be harder to make a stable deck when you don't know what they are going to play. The spins are obviously the worst part. Its rare to even see an upgraded card in the reels let alone match 3 in a row. Also as many have problably seen you can easily get Upped unupped Upped of the same card netting you nothing. Id actually wager that winning ANY card from half bloods is by far the worst rate.
Ways to change them to make them more playable: Health to 150, mark to x2 would be a nice start. Id like to see either 1/3 or 2/3 of the spins be guarentee'd upped card spins, that way at least you'll see an upped card, not have the stupid upped/unupped/upped card problem, and just have at least a little easier time getting an upped from them.