Though the routing choices it made were generally sensible - it didn't send me off on any wild goose chases during testing - its maps aren't the clearest. Those washed-out pastel-shaded graphics might look good in a product demo or in a glass case at your local purveyor of high technology, but try squinting at them when the sunshine is bouncing off a wet road in front of you and the appeal is considerably reduced. One aspect that suffers particularly from this colour scheme problem is the next turning icon. It offers a reasonably detailed representation of the upcoming junction, but again it's quite hard to see.
And, finally, address entry is over-fiddly. Instead of using the full width of the screen, the keyboard is, unnecessarily in my view, squeezed into half of it, which makes tapping out addresses and post codes a real pain. You have to concentrate hard to make sure your finger doesn't press the wrong letter and, to make matters worse, it's not the most responsive. Letters often don't appear in the address search field until a fraction of a second after you've typed them and, though this doesn't sound a long time, it is enough to ensure that when you've made a mistake you'll end up correcting more than one letter.
Price-wise, it's rather expensive: £219 for maps of just UK and Ireland is far too much these days, even taking into account the fact that it has TMC built in. If you have this much money to burn, do yourself a favour and spend it elsewhere - you can get the vastly superior TomTom Go 520 for less than £30 more.
The Navigon 5100 is a bit of a mixed bag. It does some things well, like motorway driving instructions, has a really solid windscreen mount, a system that's about as configurable as you could hope for and one that's pretty easy yo use. There's plenty in its armoury that should make it a PDA or smartphone navigation-beater. However, its urban navigation is flawed, address entry is fiddly, the maps are a not as clear as I'd have liked and it's not the best value for money.