I haven't used the DSX, but I previously used an Asus DX (which is a similarly priced PCI-E), and currently use an Asus ST (which is PCI). Both were ASIO cards with nice sound quality. I can't speak to the DSX specifically, but they've been extensively measured and reviewed over at techreport.com:
http://techreport.com/review/23358/asus-budget-xonar-dgx-and-dsx-sound-cards-reviewedI think their consensus was that the DSX was surprisingly good for the price.
To roll up my impressions with the two Asus cards I used:
The Good:
1. Low Noise Input and Output (probably the lowest noise of the sub-$500 ADCs/DACs I've used)
2. Very Stable
3. Well-behaved ASIO support* (but see my first issue under "the bad")
The Bad:
1. Asus's drivers are terrible, but there are solid 3rd party drivers out there (google Unified Asus Drivers). With the unified drivers, I have no driver complaints.
2. Both of the cards I used required separate power connectors (in addition to the MoBo slot) and they were sensitive to other devices on the same molex chain. I had to hunt around for the "lowest noise" molex. I'm not sure if the DSX has a separate power connector or not, but that was my experience with the two Asus cards I used.
I used them for line input occasionally and had no complaints (the line in sounded clean and neutral).
I've also used M-audio input devices in the past as well, and had good luck with their USB interfaces. What I would emphatically not recommend are any of the internal M-audio sound cards which (several years later) still have unresolved driver issues and stability problems.