I have an O1V that I use with my Nuendo system. I don't care much for the O1V's mic pre's; their OK, don't get me wrong, its just that I have better outboard pres, so I don't find much use for them.
So, basically, I use the O1V as a control surface for Nuendo. But as far a digital mixer's go, its hard to find a better one for the price. The O1V interfaces with an ADAT card which has optical I/O that interfaces with my Nuendo 96/52 sound card. Its a pretty hot little set-up really.