That sort of behaviour usually indicates bad data in the map, not bad routing software.
The attachment shows a really stupid route created by Garmin's software. It doesn't matter whether I try the route on Mobile PC, Mobile XT, Mapsource, my eTrex or my iQue 3600, the results are identical.
This is a continuous stretch of freeway in the middle of the mountains in British Columbia. The route on this stretch takes you off the freeway onto a dirt road that follows a valley quite a distance away from the freeway, brings you back on the freeway in the wrong direction, which requires you to backtrack quite a distance before you can make a U-turn.
The data for this map is supplied by DMTI Spatial of Canada. When I submitted the problem to them last week they agreed that the data in that section is bad. The blue section of freeway that should be a continuous line has a break in it. That forces Garmin's routing software to find a way around the break.
It makes Garmin's routing software look stupid but it actually does a great job of finding a way of getting around the break, considering the freeway is out in the middle of nowhere in the mountains.
Occasionally you can blame the routing software but the majority of the time you see a silly route it's because there's something bad in the map data.
...ken...