Well, before I had an XtremeAudio. Only had that for a couple months, since I just got annoyed with myself for buying something I thought was an X-Fi but wasn't really. The XtremeAudio is similar to the Audigy SE, as I understand it. Don't get me wrong, I loved the sound quality from it. I just didn't get the extra boost that X-Fi brings and I was expecting. The X-Fi chip is pretty incredible and does a heck of a job. It's unique to the X-Fi series (minus the XtremeAudio).
Still, the big perk I get from the XtremeGamer is EAX sound. Very nice. Another nice thing is that the sound processing is on the sound card now (XtremeAudio leaves processing on the CPU). This gives a bit of a boost in FPS for some of those more intense games (Oblivion comes to mind).
Plus, the software suite is great. They didn't do a Vista compatible one for the XtremeAudio (since it's not a true X-Fi, the X-Fi suite didn't work for it), so I'll say that I'm really liking that.
If you like your Audigy, you might be able to get away with waiting for awhile. I found a good deal on the XtremeGamer, which is why I was willing to upgrade so soon after getting the other one. I'm sure the better cards are even more impressive, or people wouldn't drop over a hundred bucks on them. But I've been pleased with what I've got.