I had a chance to play with the Prototype two years ago. I worked for a Beretta Dealer. I thought it handled poorly and the magazine ejection button popped off while I was shooting it. It was the .40 Cal version.
I assume they fixed all of the bugs by now.
I like it, I think it's a nice pistol. It looks good, it's light and low priced. I probably wouldn't ever buy one though, because I already have a 92 & a Browing High Power in 9mil Luger. A Sig 226 with a 40 S&W & a 357 Sig Barrel. Plus a few 1911s.
If I had a choice between the Px4, Browning Pro, S&W M&P, Glock & Kimber I would just choose the one with better price, warrenty, better sights, cheaper parts and cheaper magazines.