Ordered mine for a 2720 on the website today. Just to check if it was worth it I found a torrent download a couple of weeks ago. I guess that if you just need UK and "developed" Western Europe, and have 8 or 9, it may not be worth it. There are probably not enough road changes to worry about and the main difference I can see is updates to POIs (it shows my local Safeways as Waitrose now!!). Lets face it, few people bother to update paper maps every year.
If you are travelling east it's a big difference and to illustrate it, a quick tutorial. Some countries only have "basemap", some full "detail maps" and some part "detail maps". The software uses "basemap" for routes if there is no detail maps but you can't search for towns if the basemap is in use, just click on the screen for a waypoint or destination. When detail maps are available the basemap is not used for routes at all.
This led to a problem in CN9. Czech Republic, for example had VERY limited detail mapping which did not include many major roads. One route I wanted came out at about 40 miles in CN8 (only basemap available) but 110 miles in CN9 as it insisted on useing the few detail map roads. In 2008 it comes up with the correct route again. All the roads are there.
Checking in Mapsource by scrolling around Eastern Europe shows additional countries with detail mapping and by switching between 9 and 2008 they all seems to show much more detail in 2008 than they did with 9 or the basemap.
Incidentally a way to test, Mapsource (and GPS) uses Basemap for the display at small scale and detail (when available) at large scale. Set Mapsource to 15 mile and Basemap is shown. Start increasing the scale through 10 to 7 and detail maps are shown. If the maps are good more detail should show as you increase scale. If roads disappear as you zoom in it tells you that those roads were on the basemap and not on the details map - and Mapsource won't use them for routes.
It does not cost anything to get the disc but unless you qualify for a free upgrade you will need your credit card when you install it and need the unlock code.