Howard: Changes between V5 and V6 of CN Europe for countries that already had good cartographic coverage in V5 - for example, the UK - generally fall into one of three categories: updated POI detail (restaurants, fuel stations, etc.), additional road attribute information for better guidance and better decision-making on 'fastest vs. shortest' routes, and minor changes to the street detail to keep the map up to date.
Another way of putting this would be to say if you are happy with the coverage and routing performance of V5, and you don't rely heavily on the POI database to find hotels, restaurants, businesses, etc. - well, you could probably skip the update, and just upgrade every second cycle, because the roads really don't change that much.
But, for areas that had minimal or incomplete mapping coverage in V5 - Norway being an example - there might be huge additions of street detail.
Den: If you have an original monochrome SP, then don't buy any new maps for it at all - the GPSR does not have the memory capacity to hold the new mapping detail, which is hundreds of times more dense per square mile as what came with your unit. Trying to run 2004 map products on a monochrome SP would be like trying to run Windows XP Pro and Office 2003 on an old 286 processor with 32 megs of memory - simply impossible.