I use a PDA with TomTom 6 and it works just fine. I also use Fugawi Ordnance Survey digital map for countryside walking. Very handy when the walk instructions are a bit vague and you get slightly astray. You can't do that with a TomTom go etc. Also comes in handy when I sync my contacts and use the navigate to facility.
Unfortunately they don't make mine (Mitac MIO 168 Digiwalker)anymore because its about 2 years old now.