I have to say that this game has been something of a suprise for me. I had no intentions of playing it at all and thought it was just the usual Korean grindfest. Figured I might as well try the open beta the other weekend and really enjoyed it, certainly enough to pre-order the US version.
The only downside to the game for me is the early levels, I just think the prologue where they start you at level 20 for a little bit was silly (luckily it can be skipped) and the beginner island for levels 1-11 gets a bit boring, I think I must have had 5 separate quests to kill the same tree mobs so it was certainly a bit of a chore getting through this bit, but once you're off the beginner island the game really shines imho. The Pegasus ride to the mainland was a jaw dropping moment, some of the visuals in this game are stunning.
I just love the way the combat feels in this game, whether it will get old quickly is another matter, but for now I'm really enjoying it. I've been having a blast on one of the US PvP servers playing a priest. Was riding around last night looking for PKers and healing their poor victims and helping turn the tables on them

I love the way the dodge mechanic works and it means you can often stand a chance against higher levels, certainly at escaping anyway. I've only got up to level 15, but by all accounts it is supposed to get really good from level 20+ and I'm really looking forward to trying to take on these BAM's (Big Ass Monsters) which seem to be open world bosses that roam about, sounds like fun.
As for how it compares to GW2, hmm, it's difficult as to be honest as I'm really enjoying both games. I'd say for me Tera has the edge purely because it runs fine on my system where GW2 runs horribly (I'm running an old Q6600, 4gb, Nvidia 285GTX). Maybe it is the Beta client in GW2 but that is the first game that has made me think 'Deffo time for an upgrade', so for now I can see me playing Tera, certainly until I can afford a new PC to run GW2 properly. As for gameplay itself, I found the questing system much more interesting in GW2, but the combat system of Tera felt better to me.