I have an original TomTomGo, and wouldnt be without it when going into new areas.
The TomTom 300, 500 700 and One are also getting good reports.
I cannot comment on other makes, havent used them
I dont know where you got the idea that TomTom has to be upgraded each year? as far as I know it is only the mapping which needs upgrading at times, but even with the "new" maps, they are 2 years out of date.
As with any satnav , they are a very useful AID to routing, they are NOT a replacement for common sense, and using your own brain!!.
They WILL direct you down unsuitable roads, but it is up to you to determine this, by looking at where they are intending taking you BEFORE it happens, and use either "Avoid said road", or "reroute".
The great thing I have found, is it releives the stress of driving, and the stress on your other half, who is map reading,ends ALL arguments.
The voice prompts are brilliant as you never need to take your eyes off the road, which can be a lifeasver at times.
Costs are coming down all the time, so bargains can be picked up quite readily.
Have a look at the forums on http://www.expansys.com/forumoverview.asp
You will find most makes covered, and the good and bad points of each.