Having used a number of sat navs back to back while doing tech support around London (bizarely based in Birmingham) my favorite sat nav was the tom tom, after using the tom tom the garmin felt a bit clunky and the michelin navistar was awful, it'd tell you which exit to take on the roundabout just as you were going past it
Vauxhall's built in one was a junction behind in built up areas.
basically, go with well established brands, allow some adjustment time when switching between manufacturers as menu systems take a bit of getting used to.