I thought about this problem a long while back, when max payne came out.

Multiplayer fps with bullet time would be awesome. but you'd have to approach it cinematically, not use individual power-ups.

you ideally want a particular room or area to switch to bullet time when a set of certain conditions was met, say one person picks up a mega weapon, the room slows down so everyone has a chance to dive out of the way shooting as they go..

If anyone wanders into the room whilst this is happening, their time slows as they enter the room.

As long as you keep it event and locale specific, not give different time-perception to individual players (which is impossible without doing it for real) then it would work really well.

In fact it would be awesome.