The upside: mindblast + shadow word: death = nice burst on the first target.
The downside: not so great against spell-reflecting tanks with 12k+ HPs and a pocket druid/priest/shaman.
The ugly: once MB is down, you're stuck with mind flay. Nice against single targets that aren't in your face but lackluster against the rest.
The really ugly: no AoE. As a squishy team against a 5-man arena team, you either gank people one by one with relative impunity (i.e.: huge burst dmg) or you AoE them so badly that the healers can't keep up and everyone keels over. The waiting-game (aka column-humping) matches favor non-boxers and non-mana classes.

Theorycraft, of course, but the general concepts are solid, and I've tried a 4-shadowpriest, priest+4lock and pally+4mage group in similar situations in world PVP (3-5 people trying to gank me). The mages were by FAR the most successful generally as they could one-shot the healer and then AoE the shit out of everyone else. Locks are good if the other team is all kindsa confused but once you get into semi-serious/decent PVPers, you can't divvy them up and take 'em apart like you can bored scrub raiders trying to world PVP. And resto druids/shammies can heal through 12 DoTs at once with enough gear if they're able to avoid your CC.