I use Garmin (GPSmap 182) and Bluecharts and think they're very good.
I use the software on my PC and then burn the charts onto the cartridges for use in the GPS. The good news is that this way, you can get all the updates off the Garmin website (free) and keep the charts upto date on the GPS too.
The bad news is that they are expensive, and you do need to buy a lot of charts (6 I think to cover the whole of the UK). I buy them and activate them directly from the US which obviously makes them cheaper with the current exchange rate ($125 each).
I haven't notice any problems with scaling as mentioned in a previous posting and they are stunningly accurate.
My opinion of course