Omnis are extremely hard to get a great recording with, because they pick up everything. But they capture the sound in the best way, so when you CAN use them, you really should.
There's a lot of options when it comes to making a good recording, and a lot to take into account. The setup I mentioned, with the subcard and the figure 8, comes VERY close to my ideal setup in most rooms, which is why it's what i mostly used. You can tweak the recording after the fact to use more or less side, depending on the accoustics of the room, shift the image left or right, etc. It takes real world into account in a very useful way. Omnis are for ideal situations, for playback and recording. If I could, I'd build my house so that I could use omni speakers...
If you want to see some REALLY cool recording gear, check out the soundfield ST350. It's a 4 point recording system that captures everything you need for surround on all 3 axis. I REALLY want one, but a 4 channel recorder and a $5500 microphone aren't in the cards right now.