Well, i leveled my rogue to 60 pre-tbc and then to 70 as fast as i could when TBC came out. Pre-TBC i used combat as leveling, but only because everybody said that was the best. When i reached 60 i joined a PVP guild and specced sub, and then when TBC was released i stayed sub all the way to 70 (tho with a litthe twist of assassination and combat for easier leveling). And the funny thing is, i wasn't slower than anyone else! The big difference is just that as sub you have to move around more, imo it makes the gameplay more fun than just hitting sinister strike as combatspecc.
So my conclusion; Specc whatever you want, but im not sure combat is all that faster as everybody says it is. One thing i do know from own experience tho is that sub is way more fun than combat, and it makes ganking easier
/Lerbiuz, Daggerspine.