Upsides:
duo for exploring and unrivaled stealth (combined with autosneak), easy ABS wand gathering
engage pk whenever you want, be unseen whenever you want.
can locate gear to build an uber set over time, making you very hard to kill
wield a staff, high dex, makes for solid melee combined with slowing enemy etc.
madelicts, hit and run with mobs, run and duo while you heal up (plus slow)...can solo some pretty tough stuff this way
massive damage output possible, neurological is perma-lag on many races
excellent gank-squad member, more likely to attain the best gear this way
extremely survival at hero with good gear, you really have to screw up to lose pk.

Downsides:
Ranking can be tough and you are frail in the early-mid ranks, quite vulnerable until duo.
Getting to hero can be a pain with high xp penalty, even more so depending on cabal limitations, minimal solo ranking options.
Managing all the spells, appropriate sequence/timing is not as easy as you might think.