A Dual channel XLR interface with a battery compartment, external power in, AND It works alone? What more could you want? Perhaps a little more space - the back panel is very cramped.
It works wonderfully as a pre-amp. Interface wise, it's only 16/48kHz, but has 0.4ms latency at 44.1kHz, so it's pretty good for your standard user. Minijack headphone socket, two TRS outputs, with mix and level control, but all connected.
The ART Dual Pre USB is great for a two channel stand along pre-amp and recording interface, but Appication-wise it's a little limited - it's only really useful if you either NEED a two channel pre-amp (like I did for a systems installation), as there are other two-channel recording interfaces that are much better value for money. Make no mistake, though, it's a wonderful bit of kit, just not nessecarily first choice.