Hi...
I have a dedicated after market cigar lighter "plug in" sat-nav called the Road Angel, and also a Tom-Tom program on my 'HTC Kaiser' (TyNII) cellular phone.
The after market sat-nav's are OK, but the thing you should remember is that thieves like them. Because of this you have to take the device out of the car each time you park up, and carry the heavy'ish, bulky unit around with you... and more importantly remember to take your sat-nav with you when you leave home.
This all sounds easy enough but, after the sat-nav novelty had worn off, I got into the habit (as most people do) of leaving the sat-nav at home unless I knew I would need it.
So, if I was out in the car and then for what ever reason realised I needed navigation for an unforeseen reason, I was stumped - the after market sat-nav was at home.
I then learnt that I could buy a Tom-Tom sat-nav program for my "Windows Mobile 6.0" enabled cellular phone.
The beauty of having the sat-nav on your cellular is that it's always with you... I, like most people, always carry my cellular with me, now I'm NEVER in a position where Ive left it at home.
So... my advice is explore the Tom-Tom cellular "Windows Mobile 6.0" route
... if you have a cellular contract, at worst it will work out costing around the same, or, as in my personal experience, maybe substantially less, and you'll always have sat-nav with you.
M.