Stupid question time. I'm behind the trend....again

I've seen quite a bit of posts where some folks here are using IRs. Just recently I've seen it in context with cabinets as if it were a replacement or work-around. does anyone have an easy description of what you guys are doing and what it buys you?

A side question to that, of course, would be for what types of tones & music. Is it best suited for modern metal, or is it equally useful for a wide variety of things?