Well, after driving the GT-One in the WRS Week 66, I think it's a very good car.
First of all, it's fast around corners - high, medium, or low speeds. The huge downforce helps, but it really is a great cornerer.
Second, it owns the GT4 land speed record, as DE said. Meaning... High speeds are possible at, say, Le Mans - the place it was built for. The real-life GT-One, infact, has the La Sarthe laptime record...
And third, it's not too tough on the wheels and the fuelie-bar. Not the best by any means, but not as bad as Group C'ers.
And, last, it also looks great, both in Esso Red and in that Carbon Black. It's simply stunning.
And about the "GT-One loosing to LMPs and Group-Cs" - It all depends on the driver. A bad driver will loose in a Formula GT to a Daihatsu Midget. A good driver will beat a Formula GT in a Balasted Mercedes C9 Sauber.