I have Garmin's eTrex Legend and have been quite happy with it. I do a lot of backcountry hiking/backpacking in the Colorado mountains and felt at the time I bought it (about 2 years ago) that this was the smallest/lightest unit that had a downloadable topographical mapping capability. It's also pretty much water proof and even has a fairly accurate altitude capability.
Even in deep narrow canyons with limited satellite views it works well as long as it's kept in a vertical orientation...so I clip it to my backpack/daypack's shoulder straps near the upper part of my chest.
Although I also always carry a compass, altimeter, and paper topo maps as backups when hiking/backpacking (batteries can fail and canyon walls sometimes block satellite signals), the eTrex is nearly always all I need when I want to know where I am or how far it is to my next waypoint.
Definitely can recommend this unit. For more info see
http://www.garmin.com/products/etrexLegend/