Although not used with a Smith, I have used S&A magwells and have been very pleased with them.
Sometimes you may need to fit them a smidge (generally only the "ears" at the top of the mainspring housing for proper function of the grip safety), but it is a fairly simple process. You will also need to blend the inside of the mag well to the inside of the of the grip frame. It's a lot of filing and sanding, but is not absolutely necessary to do. It does provide a smoother transition from mag well to grip frame and just looks better.
If you're going to be shooting a lot of IDPA/USPSA/Steel type shooting (which can involve a lot of mag changes), I highly recommend one. If not, I wouldn't spend the money.