I wanted one of those Oly .45 conversions really bad for a while but gave it up. The general execution of the weapon seemed kind of "junky" upon examination. The magazines had a set screw in the spine for the lock up and magazine release was a big "flapper bar". Easy to get to but it changed the manual of arms. I heard you could accidently eject the magazine well adapter but I think they've since fixed the problem. Two other types of magazines from Oly. came out also. Both use the full, .233 mag well. One is a plastic magazine that has outside dimensions like a .223 mag. The other is an SMG type stick mag but it has a really funky "mushroom" top to it to fill up all of that space in the .223 well.
Like I said, I REALLY wanted one but just couldn't bring myself to spend the money everytime I handled one. You probably can't get hi-cap mags for it any more but LaFrance makes a very clean .45ACP conversion. Expensive I've heard, though.
I've just finished reading the evaluation on the new HK UMP45. Why didn't they make one WAY earlier?!? It's a futuristic .45ACP version of the MP-5. Lot's of polymer used throughout. Don't think they'll sell to civilians (but they did mention a 10 rd. magazine as an option), but let's cross our fingers!