I've leveled several rogues, rogue is a fun class in my opinion and i ALWAYS level as Assassination, it's the only spec is believe is FUN to play.
Even though Assassin is shit @ cleaving, it's probably the best at single target, Not a single time has anyone beaten me in single target as assassination while leveling.
Even though you go out of energy very fast at lower levels, IF you have heirlooms, targets should die with 1 Mutilate >.< once you get around level 50 and you have
Rupture and
Venomous Wounds you should've be able keep up abit more energy because of
Venomous Wounds.
And even at lower levels your poisons deal good damage, even without an AoE, a few auto attacks on different targets and should do be among the top in dungeons assuming you have heirlooms. (obviously there are better classes and specs for AoE, e.g. combat rogue, arms warr etc).
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Even as Arms w/ heirlooms, you rape everything >.<