I have used both tom tom software (2,3 and 5) and Medion Navigator extensively. Ignore tom tom go units, it's just tom tom software on an expensive unit which you can only use for sat nav, there are better solutions out there.
Medion Navigator is actually BETTER than tom tom in terms of how clear it is, how much speech there is and how it displays information. For example, it will stay "carry on along this road for 5 miles" where as Medion just lets you guess you have done the right thing. Medion states "At the roundabout, take the third exit onto the A57" where as tom tom states "At the roundabout, turn right, third exit". This surprised me and I find it very useful, you can have a number in your head to spot on the signs since although you know its the third exit, a lot of roundabouts are never that simple!
Medion also appears to recalculate quicker than tom tom, though this brings me to its routing...
Is it as good, worse or equal with Medion? Doing 3 routes on the two units I have here (Tom tom 5 unit and the 2004 maps version of Medion Navigator) and it gives me exactly the same route. I find tom tom much slower to learn when you know a better route, ignore comments from Medion say 3 times and it will not try to reroute you back onto your original route but guess what route you prefer to take. Medion however seems to bias national speed limit roads far too much, assuming you can sit at 60mph on them where as Medion seems to think you can sit on a 30mph road at 30mph constantly and traffic lights don't exist. Hence i find one system (medion) prefers you to take higher speed scenic routes and tom tom prefers you to take more direct lower speed routes. Swings and roundabouts..
Tom tom however wins the shows in its menu systems, full post code searching, ease of use and stability. Although medion incorporates the POI system for speed cameras etc. as standard it only comes with restaurants, car rentals etc. where as tom tom 5 comes with the speed camera database.
Tom tom 5 on a sat nav enabled PDA would be my advise, be it bluetooth/integrated or wired GPS receiver. Medion shop sold them recently for £129 and the unit is a LOT better than my Dell X5
Medion seem to be a leader in the PDA market which is fantastic for such a company.