Most people just don't like the lock. There have been cases of them causing the gun to lock up but from what I've read they were all on air weight guns in large calibers. Read more about it on the S&W forum.
Actually, some Performance Center guns don't use MIM parts. I'm hoping yours is one that dosen't. If the backside of the tirgger is "hollow" it is MIM -- if solid, it should be non-MIM. I agree on the 3" barrel. One thing that is nice about a 3" barrel -- is that you have a full-length ejector rod ensuring positive spent case ejection. The snub models have a short ejector rod -- sometimes requiring picking empties out with finger tips. The lock shouldn't casue you a problem -- I just don't like them! Nice looking revolver!
I have the exact same gun and love it. Well, I love how it looks and feels but have never fired it. It is the only gun I own that has never been fired. It was a gift from my Dad a couple years back and I think I will be keeping it unfired. I have several other Smith's that I will use instead. The flatside barrel makes for a good look and the grips are just about perfect in my hands. Great looking gun and congratulations on an awesome purchase.