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+1 for the eTrex line. Affordable, efficient, functional.
For something a little more capable, look at the 60- or 76- series. I wouldn't go less than a Cx, in fact I prefer the Cx over the CSx. The sensor package is pretty worthless - altimeter is wildly inaccurate without weekly calibration, and the compass is flaky and unreliable. I ride with a 76Cx in a RAM mount on bike and sled, >pic related.
Keep an eye out on craigslist/ebay, this line is discontinued but there's still a lot of them on the market.
Factor in the cost of maps and accessories to your budget. CityNavigator is about $100, topo maps are about $60 for a region. I almost exclusively run CityNav, only switch to topo when I'm snowmobiling. There are some free mapsets available, nationwide coverage and quality are hit-or-miss; Garmin's maps are consistent.
A decent RAM-mount setup will run about $50 if you don't have any of the parts.
I had a GPSMap 78... hated it. The new UI is awful, a lot of the useful features have been stripped out entirely or retarded to the point where they're virtually unusable. The fonts are hard to read on the screen. Data organization on the card is miserable to deal with; waypoints, tracks, and routes all go into separate data files. They're also a bit more expensive and have even more worthless features.
Learn how to use it and understand its limitations. A GPS is a navigation aid, not a replacement for common sense.
I've had:
>Trimble SLGR
>Garmin GPS12
>Garmin 60CSx
>Garmin 76Cx
>Garmin 76CSx
>Garmin 78