A personal comparison of Tom Tom, Destinator and iGo (all on a X50 :) and also my O2 XDA exec.
)
Tomtom is certainly the easiest to use and get to grips with quickly for a beginner. I found the UK maps were much better than the USA Maps (particularly on a recent trip to Arizona.)
My own personal favourite is Destinator because I find it works better when you're running other GPS applications in tandem, particularly if you are using the GPS intermediate gateway in mobile 5.
The iGO software I tried was a little clunky for my tastes and it seemed to take much longer to enter a new address, etc. Cool graphics and sounds though.
I've come to the conclusion that it doesn't much matter which one you pick. Whichever one you start off with, you grow to love it and then most things pale in comparison (or are certainly measured in comparison to that first love.)
They all get you there in the end!
Interestingly enough I still prefer to use my old X50 for navigation, because even now it works faster than the XDA exec I have for work.