I bought the ART USBmix but can't get the configuration with my guitar right. Problem is that the interface seems to send the guitar (from guitarix or other..) back and creates a feedback loop.
I have the same issue.
I've had problems with that unit trying to record while using it to monitor at the same time. It seems to use the same data channel or something. For me, I record live instrumentation on top of tracks a lot so for that reason, I don't dig it. But it does work with linux right out of the box [I use Debian x64].
I use a Digidesign MBox Mini 2.
PS. The link to the product in the main post is broken