In terms of actual "quality" of the analog output, the onboard beats every Creative soundcard except the Titanium line. that being said, Creative does a lot of work driver level to clean up the signal, and the advanced features in the driver suite [EQ, Crystalizer, Dolby/DTS/THX] can't be done with onboard audio.
Depends on your own feelings. I had a Creative Xi-Fi PCI sound card and the drivers were hell on the system. Eventually just uninstalled it and removed the card, using the onboard Via SoundMax audio and its working like a champ. More stable and less crapping out.
Creative used to make wonderful products but lately their drivers and forced "helper" programs have gotten so out of hand that I can't stand them. That and there is a known memory leak that they won't fix, every time something hooks one of those "helper" programs it cause's a small amount of memory to be allocated. The program never deallocates the memory and next call just allocates more. Each call is something like 1~2KB of memory, minor but depending on how many "enhancements" you have that could quickly get to be 400MB+. Onetime I found that it had allocated 2GB+ of memory after it was running for a week.