I voted for onboard, but now I'm not using it. Prompted by the people who are so vehemently against onboard sound, I decided to compare the onboard Realtek 883 on my K9N Platinum with my older Santa Cruz card.
As far as audio quality, I can't really be sure what the difference is. I have a bit of a tin ear, and the only thing I listen to is Windows sounds, occasional MP3s, video clips, and web page audio, so nothing but stereo, on my old SoundWorks speakers. It does seem slightly "deeper", like the sound is spread over a wider range sort of, so there's a little more detail apparently. Of course I don't know how much of a difference really quality speakers would make, or how it would sound in games.
The CPU usage surprised me. I thought the 883 was pretty good with only 3% CPU usage while playing MP3s, occasionally spiking at 9%, but the Santa Cruz is such low usage that it doesn't even register most of the time, only occasionally does the CPU usage show up at 1 to 3%. Using AMD's Power Monitor I can see that it's actually causing occasional 7% usage on a single core (2.48GHz X2).
Neither of those are something I'd spend another 100 to 150 dollars to get. Even 50 more would be a bit of a stretch for somebody who's trying to keep within a budget. Onboard quality is good enough that it comes down to the usual diminishing returns issue, whether the slightly better quality is worth a higher and higher price for each step in quality, which is an individual decision.
Of course the mainboard has optical SP/DIF output, which would be nice if I had anything to receive it with. The Santa Cruz supports 64 hardware voices too. I can't find out whether the 883 does anything in hardware.