I never looked back when I found out how easy it was to swap. It was my first mod, and the most impressive one you can do in my opinion.
Now if I had to do a lot of Interstate Driving...3:64 would be a tad bit better. But even still now when I am on roads with a 70mph limit...it's no issue.
But I also use to DD one of my built C5's with a 4:10 ratio.
Gears should the first mod for any car looking for a huge SOTP change. I then moved on to the 3SI, and then Headers....each one making the car that much more enjoyable. Milvs are next...and then I'm FBO.