For what it's worth, there still seem to be plenty of 376C's available from online retailers... for now. Prices are quite good too. Amazon had them last week for $400.
I'll be curious to hear what 620/640 owners have to say about the units, but I too can't understand why Garmin would handicap them with less functionality than the earlier units had.
I just bought an Oregon 300 and installed City Navigator maps on it; I think it'll work out well for ADV purposes, but of course it lacks the awesome hi-res display of the 620/640 or even *76 series. (On the other hand, it's a lot more compact...)
While I was researching the Oregon, I began looking seriously at the 376C, but the proprietary memory cards it uses were a dealbreaker for me. No way am I paying Garmin $100 or thereabouts for a 512MB card when a 2GB microSD card costs $7.