Mounting can be a bit of a pain. I've always put mine under the clear cover of a tank bag, but it's not very safe looking that far down while on the move. With the voice instructions on the Navman you don't really need to look at the map (if only you could hear them), so to experiment, I recently bought a pair of StudioPhonic Gold ear plug/speakers (Rebadged Etymotic Research ER6s). These combine a set of 20db noise reduction ear plugs with tiny speakers and are small enough to wear comfortably under a helmet. Note: They aren't cheap (£129.00).
I've only tried the speakers with the Navman once on the bike, so far, but I was quite impressed. They were definetly easier and safer to use than looking at a map on the move and resulted in fewer missed turns. My only gripe is that the voice directions do not include the name of the road to take (Unlike the voice directions on the Compaq Navigation System). So you have to accept on blind faith that the second exit at the roundabout is the right road, and remember that one-way roads joining the roundabout are not exits and therefore do not figure in the count. Either that or sneak a peak at the map anyway, just to confirm.
PS The IPAQ is not waterproof