I went a little different route with my GPS. I got a PDA phone that had a GPS receiver built in. It was $160 with my cingular contract so it was hard to turn down.

It runs windows mobile and that lets a few different GPS apps have a spot on there.
I loaded TOMTOM onto it. It's a good program for streets and getting around town. Each state has it's own map, so if you are traveling out of state you need to load those onto the device. It snaps you to the road line, so your GPS unit doesn't have to have the greatest accuracy. It's got voice and tells you when to turn so you don't have to have your eye on the map all the time. Also comes with pre-loaded locations( resturants, hospitals, etc).
This is really simple to use.

As far as off road or trail maps, it's hard to beat ArcPad.
Now before you even look into this, keep in mind this software has quite a learning curve.
Arcpad is GIS (Geographic info system) software so GPS is just one part of it. That is what makes this so great for trails. Data layers can be overlaid with other data layers at the same time. Think google earth, but you have the GPS so it shows you moving across the map realtime. You can load Aieral/sat images, roads, elevation (many different formats), Topo maps, even points/lines/polygons you created or data someone else created. Local gov't loves to create data BTW. Here in my county in FL, the county flies new images EVERY YEAR at 6" pixel resolution. The data is there, you might have to make a few phone calls or emails along with the web searches to get what you need. You just have to filter through and find what you want. It's a little more cumbersome than the out of the box software like tomtom, but you can get ANYTHING as far as data was concerned. I did a quick google search of CO GIS data, and came up with LOTS of info so you should be ok there.
Co GIS data(Check out the DRGs, they are the topo maps)
More GIS data
The software was made for a PDA type device but will run on a laptop or PC just fine. So if you have a GPS already and it will plug into the laptop you can use this software. Best part is that the trial version runs for 20 mins then asks if you want to save before it shuts down. So you can just restart the program and your right where you left off for another 20 mins. I use GIS software all the time so if this doesn't make sense or you get halfway into it and get stuck, post up and I'll walk you through it.