This is just my opinion, I am not affiliated with any of the below mentioned software:
I have been using navigation systems & software’s since 2001. And it’s all preference. I’ve used Rand McNally, Delorme, Street & Trips, Iguidance, Avic, Garmin Mobile PC, IGO 8 and some others not to mention.
I think the most important thing in a navigation system is,
1. Routing,
2. Map accuracy and availability
3. Ease of use.
I drive approx 45000 miles per year covering most of the east coast US. I currently use Pioneer Avic D2 in 2 car, and GMPC, IGO8, IG4, S&T 2008 on my laptop for work travel.
With extensive use of all the above software during my travels especially my trip to UK and Europe last summer, I Think Garmin Mobile PC is the overall best among this list. I do not own a carpc, use these with my laptop.
I came to this conclusion after my Europe trip and my everyday use in the US. I am not sure if the software manufacture or the map manufacture (Telatlas or Navteq, the 2 most popular) does the routing (maybe some expert here could tell me), but navteq maps are always more up to date and better routing.
I usually compare routing on at least 2 software during trip planning. (Note I update all maps as soon as they are available). These are some of the main reasons for my conclusion.
Compare to all the rest GMPC routing is always better.
You can get GMPC maps for almost any country in the world, was able to navigate using GMPC from London to Birmingham UK and Paris to Switzerland via Brussels and Germany, (my first time to UK & Europe) with no significant trouble.
My best feature is Custom POI. Arm with the latest Speed & Red-light Cam dBase (thanks to pocketgpsworld) saved me so many times, also being vegetarian armed with restaurant list (thanks to POI Factory). I was alerted when I was within 5 miles of a vegetarian food place.
GMPC does not have the best map graphics (like 3D buildings, Highway Lane count, 3D freeway exchange/junction view etc) of these software On the other hand GMPC Online features like weather, hotel rates, fuel prices & flight status all work if you have internet access. GMPC have some cons like jerky tracking when using 1Hz gps receivers but to me those feature are secondary.
I would like to hear others opinion that use navigation systems extensively.
Hope this helps you in making your decision.
Thanks for reading.