I own a Garmin unit and am extremely satisfied. I like Garmin over the Magellan brand. I have the GPS II Plus which is becoming a slightly older model. It does not have the viewable street maps because I didn't want that feature. The integrated street maps for GPS units aren't updated frequently enough. You are better off using a standalone GPS unit along with a dedicated street mapping program like some of the Delorme products (Street Atlas.)
No matter how you use it, you can gather your waypoints in the field then plot them later in any sort of program. In fact, I recently produced a street map for our local park district and hundreds of the street were plotted from the GPS unit. There are standalone programs that will allow you to download the raw waypoint data from your GPS unit and use it as you see fit.
One way to increase the accuracy of any GPS unit is to post-process the data. It's an expensive proposition that's usually reserved for high end surveying stations. Delorme sells a small GPS unit for about $100 that takes advantage of the same technology. It basically 'corrects' the GPS data from atmospheric disturbances and time delays, etc. For $100, it's a great deal to have such accuracy (about 9'.)
All in all, the GPS unit isn't going to be accurate enough for you to actually use in a surveying sense. But it will allow you to generally plot larger tracts of land. When you are using the GPS unit it will be acquiring tracking data as you move about. The problem is that it doesn't track every inch that you move because of memory limitations. So the raw tracking data usually won't be fine enough (what is called granularity) for you. However, you can overcome that limitation by plotting specific waypoints at spots on the ground where you need a accurate point plot.
Let me know if you have any other GPS questions. I use mine extensively for backpacking & mapping and extremely familiar with the technology...
I have the DeLorme Street Atlas 2003 and their Earthmate GPS that I use with my laptop. I plot out my course ahead of time with subject & comparables. It's great! I don't know what I'd do without it since I moved into a much more populated area than what I was used to. It gives estimated times and mileage for each leg of the trip and even has a feature that tells you your next turn will be xx distance.
They have just come out with a newer GPS that is smaller and faster than the one I have. If you already have the laptop you can get the new improved GPS and the mapping software for $130.
I find that it is accurate about 90% of the time. Sometimes it will try to send me down a "paper" road. When I go into some towns, the line is slightly off from the maps, so it can be a little more challenging to figure out.
I use ACI software and the DeLorme integrates with it. It will automatically make your maps showing locations and you just copy & paste it into your report.