I have yet to be in it, but have talked to approximately 5 different people that have. All have commented on more power than standard 350's and just smoother and quieter. I am sure others that have been in them will speak up as well.
I have one in my new MB and love it. More power than my last Ilmor 5.7 and sips fuel. I have about 25 hours on mine and every time I have gone to fill it up I'm surprised how little fuel it has used. Seems fairly easy to service with the exception of draining the manifolds when installed in the MB. Hopefully Indmar will come up with a solution to that over the winter.
I'd suspect it performs better than a 350 Chevy but a far cry from a 454/496 or 460. After a few years when things may break repairs will be more expensive and time consuming than a standard big block. Is this engine a 4.6/5.4 based engine with a larger bore and/or stroke or is it an entirely different type of engine?