I already own a Creative X-FI XtremeGamer costing about twice as much as the Xonar DS. I started getting the dreaded 'Creative click/popping sounds' after updating my ATI video drivers, and needed a replacement. I plumbed for the Xonar DS.
The Xonar does not have as full a sound as the X-FI, but is certainly almost on a par, and you typically won't notice the difference unless you are a big Dolby fan (the Xonar doesn't have Dolby - but to be honest, I don't like the sound that Dolby gives and never used it on the X-FI) and you generally won't spot the difference during gaming. Better still, the Xonar doesn't have the bloatware-pretending-to-be-drivers that the Creative cards tend to have.
Bottom line: If like me, you can't stand poor quality on-board sound (or you have good quality speakers) and want good, no frills HI-FI sound then you want a Xonar DS. Its currently the best bet on the sound quality vs price front, and you're much less likely to have hardware issues with a Xonar than you are with Creative. Its also worth mentioning that unlike CPUs, memory and graphics cards, a sound card may seem like a bit of a luxury, but they last forever simply because they dont date like other hardware. Quality sound for as long as you have a motherboard with a PCI slot in it, and all for the price of one new game DVD. Can't be bad!
*** Update Jume 2010 ***
There's a minor issue in the review above. Although the Creative X-FI sounds better than the Xonar when listening to music/videos, this is NOT the case when playing games. The X-FI doesnt actually support all its high end sound features during gaming applications, so the XONAR actually sounds pretty much identical during gameplay.