p90's are great. They basically sound mid-way between a (fender-style)single coil and a humbucker. They have a bit more power, and are warmer than single coils, but have a bit more top end sparkle (than humbuckers) like single coils.
Only real downside is that they aren't noiseless like humbuckers. And aren't really as powerful, if you're after, say, modern metal.
that's a simplification, but i guess it'll do.