There were some issues with the PN-20; evidently it is very slow to draw and update the screen. Everyone says the PN-40 is much faster, so that's probably a better choice. If you want custom maps and ariel imagery, this is really the only option for a consumer product. The screen seems quite small however, especially considering the type of imagery it uses.
I haven't used one myself, but lots of discussion over at GroundSpeak:
http://forums.groundspeak.com/GC/index.php?showtopic=208027
http://forums.groundspeak.com/GC/index.php?showtopic=208598
I have a number of GPS'es, including a Garmin 60csx and Oregon. The 60csx is considered the "gold standard" by many people, and it's certainly a nice unit. But the weak part is the tiny low resolution screen which can only display 256 colors. For all you 60csx fans... I plan to sell mine as soon as I can get my act together... will list it here when that time comes.
See if you can find a store which carries several different models... I think REI in Marlton has a pretty good selection. It would be best to play around with them personally to see how you feel. The Oregon is certainly worth looking at too.