Rogue: 2 piece and 8 piece are both quite good, especially 8.
Hunter: 2 piece is quite good, and 8 piece is ridiculously good.
Paladin: 5 piece gives a great boost to damage and healing.
Mage: 8 piece 10% chance for instant spell, not sure how good that actually is for PvE overall though
considering it also uses up a GCD and mana.
Druid: 3 piece mana regen is quite good and 8 piece 3 more sec to rejuv gives a nice boost to healing overall.
Shaman: 3 piece 30% more heal from chain to secondary targets, seems quite sick, and 5 piece 3% crit is quite decent
Priest: 3 piece mana regen is quite good and 8 piece and free rank 5 renew on greater healing is quite OP.
The problem with the casters mage and lock is the lack of hit, which means that the sets aren't that great and can be replaced by blues that are addded in later patches, sadly
Mage T2 is AMAZING for PvP though.
I would say that priest and hunter are the biggest winners overall.