GPS/NAV units are kind of a hobby of mine & have been for years (my first unit had a black & white screen). I sold Garmin units at one time & they are my favorite units at this time.
I have used/owned/tested most of the units on the market at one time or another (I actually have 3 different NAV units to choose from in my truck right now). To me, the routing engine performance is the first thing I look at when choosing a GPS. I know how frustrating it can be to own a unit that has sub-par routing. At one point, I owned a Pioneer Avic Z1 built in double DIN unit. This $2200 unit was an absolutely AWESOME entertainment center, but the routing engine was awful (I ended up selling that unit on ebay).
We where out scouting colleges for my son to attend & that POS was routing us to one we had never been to. At one point, it routed us off an exit that came to a T where you had to turn either right or left. Although both my Wife & I where pretty sure the school was to the left, the unit insisted we where to turn right which we did. About 5 miles down the road, the unit piped up & said "make a legal u-turn". We where on a 4 lane divided highway at the time...... I had to go another 5 miles or so to find an exit where I could turn around, just to end up driving right past the exit where we should have turned left in the first place. No NAV unit is perfect, but this kind of stuff happened constantly with the Z1.
Another time it gave what seemed like a very round about way down the East coast to another school, so I pulled up Google maps on my PDA & had Google plan a route. Google's route was 89 miles shorter & made perfect sense.
~John