It's sort of the gospel here that omnis will only sound good in a near-perfect setting ("Omnis indoors are fine when all the variables are perfect. Quiet crowd around you, mics are in the 'sweet spot' in the room, and the room itself has excellent acoustics," as Belexes put it). In my experience, though, omnis can sound excellent in less than ideal settings too. One important factor, as Cheesecadet said, is proximity. In rooms with poor acoustics, for example, getting closer increases the ratio of direct to reverberant sound and makes the room much less of a factor (boundary mounting can also be useful in that respect). Chatter is also often minimized as an issue close-up, at least for fairly loud shows.
For much of the music I record (mostly jazz), the low-end isn't hyped like at most rock shows and the (generally better) low-end response of omnis is a real plus. By contrast, my card jazz recordings have often sounded a little thin bass-wise.
Omnis have their plusses and their minuses. So do the other polar patterns. Like I wrote a few posts up, a lot depends on "type of music, crowd, venue, location, etc." Crowd and venue are usually bigger issues at rock shows than at jazz or classical concerts. Bottom line: there are no one-size-fits-all answers to this question. My two cents, YMMV, and all that good stuff...