I suppose the question applies to Linux in general but what have user's experiences been with Asus Xonar U3? Does it improve the sound quality of onboard or the generic drivers cripple it?
I have used this device in Ubuntu 11.10 and my experience is that it does improve the sound quality and adds a lot more power to the output (Acer Extensa laptop). Of course the actual improvement depends on the quality of the onboard sound chip.
Unfortunately the Xonar U3 Audio Center SW doesn't support Linux but the device is usable also without it.
My experience with the Xonar U3 and 11.10 on a Samsung RV511 laptop has been very good.
Installation is dead simple, just plug in and go to Sound Preferences (right click on Sound icon) and the hardware, input and output tabs selecting in turn the USB Advanced Audio Device. And there you have it, a noticeably improved sound output, haven't used the Xonar with a mic yet so I can't comment on the input capabilities of the Xonar.
The Xonar can also be accessed through alsamixer, not sure whether this is any different to using the Sound Preferences (probably not).