I just got back from a few days' hill walking in the English Lake District using Maep with the OpenCycleMap overlay for contours and path info.

I pre-cached the Maep tile data at home while I still had an internet connection, then put the N900 in offline mode for the rest of the time (very poor reception up there, if any, plus I wanted to conserve power).

With no internet connection, the GPS lock was extremely good although it took a while (up to a few minutes) to get an initial fix. After that, it was great. Hardly needed the map & compass at all other than to verify my route at the start of a day. (OpenCycleMap is great but I don't trust it with my safety).

Agreed, the Nokia Maps application on the N900 is rubbish compared to 'real' SatNav. But:
1) the GPS on the device is pretty decent.
2) it is perfectly able to get a lock without a network connection, it just takes (up to) a few minutes.