It makes for an excellent and respectively cheap time filler. There's generally always *something* to do, though it's not always an interesting or fun something. $15 a month is pretty much the cheapest you're going to get entertainment.
A lot of people seem to enjoy the PVP, I couldn't get into it though. Basically you play a quick game to see who can take away control of the other character(s) longer in the arena, or play on one of a number of static maps with a specific objective. Unfortunately, last i left, battlegrounds(the latter part, and the part i enjoyed in small doses) offered totally worthless rewards and you were forced to play the arena, where you will lose unless you and your partner(s) have 56 forms of crowd control.
PvE is a little less annoying, however you will run out of relevant non-raid content after a month or two of being level capped. At that point you can totally give up and do battlegrounds for crap pvp gear so you can do arenas for less crap pvp gear, or roll an alt character and start the whole process again, OR join a guild that runs 25 man and/or 40 man instances for raids, be there at x time on x nights per week, and basically do PVE battleground with ginormous targets and dancing through various raid encounters. The raids can be fun, the first 15 or so times you kill something, however you will kill any given raid boss way more than 15 times gearing everyone up. Maybe add a zero to that number.
All in all, it was a very inexpensive way to spend three years of my life, however i dunno if i'm too keen on picking it up again, given that the formula doesn't seem like it will ever change.