Originally I tried to buy the Nokia 770 on it's own from another supplier because, following a price cut to less than £80 I figured it was good value for a networkable Linux PDA. Unfortunately they let me down, so next stop was Amazon to see if I could source one from them. They only had the "Navikit" pack available for nearly double what I was going to pay but, since I don't have sat-nav already, I figured I could justify the extra expense to get a large memory card and the GPS receiver. So I'm using it in two modes: as a PDA/wireless-tablet and as a sat-nav device.
As a PDA it's not that good - a Palm or Windows PDA would be better. But it it very expandable and the media players are usable. The downside is that this uses RSMMC or MMCmobile cards rather than larger/cheaper/easier-to-get SD or variants thereof. Plays a mean game of chess too.
As a wireless tablet, it's pretty usable (with restrictions). The screen is quite clear, and it starts quicker than most PC's - ideal for those "I just need to check the news/weather/road-report" moments. I've also found that the wireless hardware in it is very good - better than the Netgear and US Robotics cards in my desktop PC. The webbrowser is limited by the lack of screen size and keyboard, plus (currently) the lack of support for "essential" web technologies such as Java and Flash.
The GPS receiver is a Nokia branded one, which is small (about the size of a cigarette lighter) and appears to use the same rechargable battery and charger as the '770 itself. The sat-nav software is supplied by NavicoreTech and is pre-installed on the 1GB RS-MMC card supplied in the pack, (so technically it's plug-in and go). Also in the pack are a windscreen mount for the '770 and a cigarette lighter power lead for the '770.
I'd strongly recommend visiting NavicoreTech's site and upgrading the software (using a Windows PC usually) as the later version is noticably better than the default version supplied. You can also then add extra content such as other voice models (the "Afrikaans" ones are my favourites at the moment) and extra points-of-interest such as speed camera locations.
In use the sat-nav, (which has distinct drive, walk and cycle modes - and I've tested the first two in that list), is quite usable and takes about 30-120 seconds to obtain the location when first switched on. Instructions and screen display are clear, although a little more volume from the '770 would be nice, (time to use an FM Radio sender?), and the software handles dynamic rerouting (when you change your mind) reasonably most of the time. Downsides are that the software sometimes needs more than one attempt to start and occasionally it also can get very fixed in it's thinking, preferring to send you back on yourself rather than generate a simpler route ahead. The GPS receiver doesn't seem to work at all in most multi-storey car parks, which can be a problem.
In conclusion I'm quite happy with my choice. I figure that a dedicated sat-nav costs £120-190 and can't really be used for anything else, (although I know some have MP3 players now), most of them being car-only devices. On the other hand, my Nokia kit will browse the internet, play MP3's, play videos as well as being usable as a sat-nav/mapper for when I'm either in the car or trying to get to an urban location I'm unsure about on foot.
At Nokia's original price (£250+) I wouldn't recommend this, however at the current £100 discount this is very good value for money indeed, even allowing that the '770 itself has been replaced by a newer model, (and the NavicoreTech software and GPS also works with the newer model - Nokia 800 - so there's an upgrade path if that newer device becomes cheaper).