I have a Garmin Street Pilot 2720 which has served me very well for several years.

I have actually found the internal database to be quite good, and use it quite a lot.

Is it perfect, no. In fact I've found when some areas it is worse than others. In the Southern CA area, it does pretty good, but we have a lot of freeways, and most places will take you freeway, and minimum turns beyond that.

In areas where there aren't as many, they I've found it to be a little less accurate.

GPS directions should always be tempered, if possible, by looking at a map. I don't know if any of the units successfully have a way of routing specfically for a bus or RV.

I've been happy with my Garmin, while my friends Magellen sucked (main user interface issues). As you might guess, routing decisions are not trivial. What's obvious to a human is not at all obvious to a program looking at hundreds of streets possible in getting someone from point a to point b. I don't know for sure, but I would assume that the primary information available within the routing software is just the street name, and perhaps posted speed limit, and whether a road is major or minor. My unit seems to know that much. But, it probably doesn't know much between varying grades of major and minor, nor does it probably have any info about how "skinny" a road might be. Some units I believe have access to underpass information, and might route depending upon height restrictions.

Programming-wise it's an incredible task, and the units do "pretty well" considering.

Ray