I had one for a while and it was fine. I only took it back cos I wanted the NI Audio DJ 8 instead cos it had support for Traktor Scratch. I don't really use Traktor scratch though cos I couldn't be bothered settting up my laptop all the time. Generally though I thought it was good, but I do have my reservations about USB and using large numbers of VST's and process heavy tracks. Ideally a PCI card is much better imo. The internal routing and software of the NI stuff is not as good as the M Audio stuff. With MAudio you can assign any output to the headphones and they have a decent software mixer. NI doesn't have a interanal software mixer. Read the NI forums
HERE to get an idea of the issues people may have had.
Now then now then.....a far better buy in my opinion would be the
MAudio fast track ultra. Not much more and the spec shits on the NI audio control 1.