The "new" maps were out of date - road alterations which had happened 2 years previously in my home town weren't on there yet. We found various other places on our trip where it seemed the road layout had changed since the maps came out, too.
Maybe I did something wrong somehow but it wasn't a very good advert for paying for maps. I've never used OSM on a drive but it's very good for walking in my local area as it has pretty much every footpath/cycle path (unlike Google Maps, which is very road-based), so I'll try it next time.
As for that older TomTom, I replaced it with a newer model because the battery life had gone right down and the touchscreen was failing, but the new model touchscreen is also very unresponsive, it already needed constant charging even when it was brand new, and the interface and directions aren't as clear as the decade-old model. I won't buy TomTom again.