If you're very unsure and have 2 hours and 40 minutes, here [http://www.youtube.com/watch?v=e1ls5L4blYM&feature=g-u&context=G265d977FUAAAAAAAAAQ] is a pretty informative discussion between three veteran MMO players. In short, they say:
It plays very much like WoW with really only 1 new machanic for one specific class (IA/Smuggler);
Everything it does that WoW also does, ToR does as well or better;
The stories (each class has its own story) are in general very good, make more sense and are more interesting than "pretty much any other MMO" they had played (rough quote, listened to it last night);
PvP is not very good right now, but the big class-balance issues should sort themselves out when people learn what they're supposed to do - AoE classes rule the day because nobody knows who to kill, so they just randomly hurt anybody and they expect that to sort itself out once people learn more about the classes and which need killing, giving single-target classes a stronger position - and the "you always play Huttball" will hopefully sort itself out once more people PvP;
It's quite buggy, some of which have been around since early beta;
There's a lot of instancing, making grouping useless/bothersome too often (but far from always) while questing;
That is solved by having companions, squadmates like in most Bioware RPGs who can also be sent off to work on professions - since you can only bring 1 with you and you get 5-6 of them, you can have 4-5 people gathering or crafting while you quest (or you can do it yourself I think);
The companions - like in other Bioware RPGs - have their own personalities, chip in on conversations with NPCs and from time to time want a private chat if you piss them off, which the people in the podcast found great and immersive;
You get your own ship (woo!) but you can't take other people on it (boo!);
Early dungeons are of wildly varying quality (the first being gailed as brilliant, the next 2-3 being called boring and unimaginative);
It's the best, most refined and most content-laden MMO on release they've ever played.
Sounds one-sided I know, but if you listen to the podcast they have trouble to keep from praising it through most of it. Kind of in closing they said that it's very much worth the buy, but nobody can say if it's worth the subscription yet.