My opinion is that it is possibly the greatest J frame ever made.
I had some trouble with it initially, but S&W helped me out. No big deal, as I've had some trouble with my Gp100 that Ruger helped me out with too. I always give manufacturers a chance to make things right, and then test thoroughly afterwards.
I tend to pocket carry it, but have taken it around town in an IWB holster as well. In point of fact it is in my pocket right now, using the same MIKA's pocket holster that my 442 broke in for me. I don't mind the weight.
The pro model featured here has no internal lock, sports a fluted barrel and tritium night sights, and is cut for moonclips. I use regular old HKS speedloaders (which the altamont and badger grips allowed for) since I found the moonclips were too fragile for me to be comfortable with on a ccw basis. The badger boot grips give me just a little bit more vertical space for a good grip with the pinky.
Keeping it loaded at the moment with Buffalo Barnes 125 grain XPB bullet (1225 ft/s out of a short barrel) and Underwood's 158 grain gold dot hollow point.
I was in the market for a 640 or an Sp101. Both are great firearms, but the Sp101 was just a bit too bulky for my needs at the time. The 640 had the right mix of power, strength, and concealability that I was looking for.