My friends and I installed a TransGo shift kit in my mustang. We used the school auto shop so we had the help of a lift and some other tools. The snap rings are a pain to take out, that was the longest part of the entire installation. I don't have that much mechanical experience but I use common sense, and I thought I was always thinking that we were going to break something. We had a TV in the garage and we were going along with the manual and the video which made it a lot easier. Overall the entire installation took 3 days, working 1.5 hours a day. I cleaned everything so it was spotless with brake cleaner, that took a good portion of the time too.
If you have all of the tools then do it yourself. It's not hard at all and you will learn how some of the transmission stuff works. If you're looking at having a mechanic install it then figure about 3 hours of labor, the cost of new transmission fluid (I used Valvoline Mercon V which was $4 a bottle, I think it was $40 total), and a new filter (if you need one).
When I first drove the car with the shift kit, I had almost no mods done to the car. I didn't feel neck snapping shifts but the shifts were definately quicker. After the 4.10 gears were installed there was a huge difference. The shifts are really hard at full throttle. On some days I can chirp the tires on the 2-3 shift.