I was on a free trial of BC and got my Blood Elf Hunter up to 23...of course 5 of that 10 day trial was spent downloading and updating the game on my pc.
I got another free trial...just regular WoW and I am trying to get my Night Elf Hunter as high as I can get em'...1 day in and a lvl 10.
I only played 2 hours last night. I need to get BC, WotLK and CAT...someday.
Here's my honest take, after having spent the money already.
Unless you really, really, really want to play as a Blood Elf, don't bother buying the Burning Crusade until you're getting close to level 60.
Unless you really, really, really want to play as a Death Knight, don't get Wrath of the Lich King until you are getting close to level 70.
Unless you really, really, really want to play as a Goblin or a Worgen, don't get Cataclysm until you are getting close to level 80.