Chris,
I've used a Garmin Nuvi GPS unit for both driving and walking and it's worked really well so far in both applications. I've found that if I first get a "lock" in a location with a good view of the sky, the unit seems to be able to still function even if I venture between buildings.
However, I have to agree with Nancy that the battery life is one of the weak points. My GPS unit is a few years old, so the newer units may have larger capacity batteries installed? I tend to use it sparingly when in pedestrian mode and turn it off if not required. I can usually keep it working for a full day of touring with that method.
Using one of hand-held models probably wouldn't be as suitable for a combination of driving and walking. As the others mentioned, those models are more intended for hiking and outdoor activities rather than driving and the installed data bases will reflect that (ie: they probably won't be able to provide turn-by-turn driving directions, place of interest, etc.).
Be sure to buy a model with European Maps installed. If I were in the market for a new GPS unit, I'd be looking at the nüvi 1370T, 275T or 775T (the model I'm using now is equivalent to the 275T).
Although GPS units sometimes make "routing errors", I won't ever travel without one now. If you're driving, I'd suggest taking a good Map as a backup (I usually use Michelin).
Good luck and happy travels!