i want to get the felguard at 51 because i hear its great. but should i spec with destruction or affliction until 51 and then relearn? or is demonology good enough for me to do well just by using that?

i already know im getting felguard. but wich tree should i spec w/ until 51?

Actually, you get the felguard at lvl 50 if you have not spent any points on another spec. I am a lvl 51 lock and went demon all the way and loved it. I know there is more dps with other specs but I never had any trouble in instances (with good groups) or grinding by myself. The VW does fine for grind tanking and you usually use the imp in group anyways for the blood pact stam buff. Just like anything else in WoW it is a preference thing.

Good Luck!

...In addition I have to tell you my first experience with the felguard...

I had just hit 50 and summoned my felguard when my buddy asked if I could run him thru deadmines. I entered the instance and set the feguard on aggressive to see how he would do. That maniac just wiped through those miners like butter! All I did was walk around where I wanted him to go. It was the funniest thing to watch. I never used an attack or DoT until we reached the first boss and then all I did was hit him with my staff while the felguard did his business. When I reached Van Cleef I decided to see how he could do on a lvl 20 elite by himself. I just sent him in and he finished off VC and all of his spawns without a single attack or spell from me. Now I know there is a HUGE difference between a lvl 50 felguard and a lvl 20 elite, but it was still a great first impression of my felguard.

Well i went affl and once i had unstable affl i went to demo and got instant summon...its very helpful for pvp

affliction is good unless you want to be those guys on http://youtube.com/watch?v=wI4qcEFvrtI and get like 10k shadow bolt spec destruction its good : )