I'm still chipping away at Mines of Moria content, but I pre-purchased the expansion for the rewards (particularly the +25% XP bonus)...its nice knowing that I have the entire game to play pretty much indefinitely for free.
LOTRO will never be my primary MMO, but I really enjoy how expansion middle earth is... you can spend hours just exploring a single zone. Mobs are hard, quests are difficult, crafting takes a long time, travel is long... its a very immersive game.