Buying Guide

What Are your Needs
Before you even start to browse the web for the latest SatNav systems take a moment to think why you need a unit and what you'll be using it for. There are various different types of system setups that are listed below
In Car Fixed
These systems are often integrated with the cars entertainment systems and have large colour screens. Unless you like being mugged while buying a car, or the dealer throws it in FOC then avoid these like the plague. They are often difficult to use with poor data entry (very few are touch screen) and the maps go out of date quickly and are rarely updated.
Portable
Probably the most popular system and can be easily transferred between vehicles and are normally held in place with a windscreen sucker mount. Makers such as TomTom, Garmin and Navman have become familiar names.
PDA Based
PDA is a alternative to portable systems. PDA's have always been a curious gadget with many owners not really knowing why they carry them. PDA based system has been somewhat overshadowed by the Portable, however there are still excellent systems available and they often have cutting edge features ahead of the all in one units.

Other things to consider when choosing Sat Nav are listed below
Accuracy
The current American GPS system should allow systems to navigate to around 10 meters accuracy, it's vital that your chosen system is as accurate as possible you don't want it thinking you are further down the road than you really are or you'll make a wrong turn.
Reliability
Sounds daft but reliability should not be assumed. Check your unit has the latest chipset (SiRF Star III) the latest chipsets are far more accurate and less prone to struggling amongst high buildings in town, or when roads are overhung by trees. Another key area here is to check if your car has a heat reflective or heated windscreen, if it has then a unit with a built in GPS antenna may be trouble as it won't be able to get enough signal strength. .
Speed of Calculation
Two things to test for here, first is the TTF or Time To Fix, you want a unit that quickly acquires enough satellites or "lock" to start navigating, otherwise you'll sit frustrated in car parks waiting for your SatNav unit to wake up! ..
Interface and Audio
The interface on a GPS navigation system is critical perhaps more so than on any other form of consumer electronics as it will be used while you are in control of a vehicle at up to 70mph. Therefore you are looking for something simple, clean and effective, if it looks like a game of space invaders or that only a geek could operate it steer well clear.
Power
The last major category of note is power. Sounds basic but you need to check that the power lead will work in your vehicle and that it will reach, some system have an amazingly short power lead.
Lastly always try before you buy so you get the right unit for your requirements.

Personally I wouldn't bother with sat. I am a delivery driver and over the years I have delivered into just about most places in the uk and I have always used my own system of pen and pad using my own symbols and a form of short hand. I can deliver to some place in say, London (or any where else) and can land on the doorstep. I maybe old fashioned but I think that it's a bit like when they allowed us at school to use calculators we tend to forget how to add and subtract. So when the satnav goes on the blink, the average driver will never find there way home.

i understand it is your personel view bill, but the post is a guide to what to get.
personnelly for me it has been a godsend, directing me to numerous different places in a relaxed manner,getting me in the correct lane approaching a large roundabout or multi exit points and in some cases showing/taking me a quicker easier way,thereby saving me money. i'm very glad that i have one and use it alot.