How I use GPS in MTB
1. Find Trail Head - On new trails located by reading books or web descriptions, it's often hard to even find the trail head in remote areas. By reading the directions and looking at online maps, I can usually figure where the trail head is located on the computer maps. I usually do this by using Garmin's MapSource and flipping back and forth between the street maps (MetroGuide) and Topo on the same area. Downloading the MapSource street maps helps in navigating ot the trail head.
Software: Garmin MapSource MetroGuide (All of US for ~$85)
2. Navigate New Trail. - Same thing as above. First, I use online maps and Garmin's Topo to figure out where the trail juctions are located and waypoint them. I look at available online maps of the park from various sources and use peaks, rivers, and countour shaps to figure out where the trail junctions are located. I'm usually right to around 100'. I create routes by connecting trail junctions. Having a mileage count down to the next waypoint is reassuring that you're on the right trail, as well as a motivator. I mostly use Garmin's MapSource Topo for trail navigation, downloaded to my eTrex Vista.
Software: Garmin MapSource Topo (All of US for ~$85)
Topofusion or other satellite software may help here too - haven't tried yet myself. But others said they can find trails from satellite photos and waypoint critical junctions from the satellite map.
3. Points of Interest. - If you need a replacment bike part, money from an ATM, after ride eats, or gas, it's nice to have the Points of Interest on your GPS. Shows the nearest feature of interest (bank, dept store, restaurants by type, ...) along with address and phone number to check if they're open. Like a Yahoo Yellow Pages in your palm, sorted by your location at that instant.
Software: Garmin MapSource Metroguide
4. Recording Rides - For personal use, I use the Garmin MapSource software as it's easiest and fastest. I can create new waypoints and correct waypoints in relation to recorded tracks. Tracks of previous rides help in planning new routes. Either MapSource Metro or Topo will work for this.
Topofusion also seems to have some nice features from what I read in Rich Owing's book
5. Showing Ride Profiles, (3D) Topo Maps with Tracks. For planning or showing ride profiles, National Geographic has worked best for me. The automatic profile software has improved and projects climb within 10% of the actual ride, as good as you can get anyway. This is important to make sure my planned ride is not beyond my ability and I can get back before dark. National Geographic has the nicest profiles, so most people show post ride profiles using NG. You can get a great 3D package for only $25 more. It works with the eTrex line directly using the serial port.
Software: National Geographic TOPO! (Each state costs ~$85, 3D another $25)
Topofusion may also offer some 3D maps
In general, I find the plots of tracks on satellite maps are not as readable or pretty as the Garmin or NG software. But Topofusion has some nice 3D plots, from what I see in Rich Owing's book.
Also, you need the Garmin MapSource software if you want to download maps to your GPS. I flip back and forth from street to topo maps on my computer and GPS. With 24 MB on the eTrex Vista, I can store all detailed street and topo maps of the entire SF Bay area. If you have the 8 MB (non-color) Legend, you may need to carefully pick your maps to download.
Like Formica, I suggest you first get Rich Owing's book and see what software can help you with the functions you find most useful. His book is very current and can direct you to the free or low cost web software (like topofusion and many others) coming out now and the various advantages of each.