NAV3 was a complete re-write of the code, following the settlement with Microsoft (over alleged copyright or patent infringement with TT's use of the FAT32 file system)
There is a LOT to recommend it. It is faster in a lot of ways, especially on route planning, which is blisteringly quick when you compare it side by side with an older NAV2 model.
Most people would also say it has a more modern, cleaner look than the old software, although I think it's a bit too garish and "cartoon-y" (a bit like a Garmin!)
However, it's fairly obvious that it was rushed to market long before it was ready, and the way the software design has been subcontracted out means there is much more of a "kit-based" approach to the different parts of the code.
It's only a personal opinion, but my gut feeling is that the original core software was somebody's baby that they nurtured and honed to be as lean as they could get it, while the new software system is just bolted together from a kit of parts.
That's not to say TomTom aren't bothering to try to improve the new system, far from it.
They are feverishly working on various bits of it, but I think they found many of the problems were far harder to crack than they first imagined and also in these tough economic times they have simply no longer got the man-power to sort things quickly enough.
Part of that has been demonstrated by the way that certain functions were not available at launch, were then promised as a free update when they were ready, but that eventually never appeared, or did so in a much reduced state.
The itinerary planning function is a perfect example of that.
Unfortunately, as a company they are not very open, and they tend to cope with criticism by simply ignoring it. So after two years we are still none the wiser WHY some of the features have not returned.
I use both NAV2 and NAV3 TomToms on a daily basis and I would LOVE to only use NAV3.
On the whole, the units are sleek and well-designed, and their faster processors and better screens mean a lot should be possible.
So it pains me when I know I can still tap through the menus quicker on a 5-year old device than I can on a brand new one.
It pains me when I cannot search for a POI that's more then 50 miles away from me without knowing a city it's close to.
It pains me I cannot add extras and add-ons to the system and I can't read a text doc or listen to music on it. All of these were stripped out in the interests of simplicity.
But please don't take my word for it... You may find you love the new "look and feel" and don't feel a need for the things that are missing. If you an get a chance to actually have a play with both old and new types in a shop, please do!