I use a backpack Trimble PRO XRS. Not a cheap unit but very good in all conditions. The fancier units incorporate what is called "Real Time Differential Correction" which simply means they descramble and refine the signal and position fixes as you take them. Generally, that is done using a known point source as a correction, which in coastal conditions is the Coast Guard beacons. These emit a long ground wave signal that transmits thousands of miles - I can use a beacon from Alaska (but actaully use one from much closer). To get really accurate you can further refine the position fixes by a process known as Post Processing, which takes the position fixes and runs them through another programme to reduce errors even more.
The backpack unit is quite capable of achieving survey grade readings, but it requires a lot of fixes to do that. Typically I can work at the plus / minus 1 metre level easily. In reality, when I am mapping creeks, land features, power poles etc, I tend to work at the plus minus 5 metre level, since that is sufficient to portray the features in a reasonably accurate plan (but not to survey grade). Note that the accuracy levels set may in fact be much better in ideal conditions, but the user needs to specify where the accuracy threshold will "typically" be, to avoid false accuracy claims.
The software allows me to see the standard deviation of the fixes collected, and I can edit those to eliminate outliers from the overall cluster, which improves statistical accuracy further.
There is a temptation to believe that the handhelds give great accuracy. To be certain you need to really understand how GPS works, including satellite geometry, signal transmission technology, and then translate that to changing field conditions. Hand helds have a reasonbable chance of accuracy in open conditions (clear skies - no obstructions). They degrade pretty fast once obstructions come into play since these tend to bounce the signals around (multipath errors)which in turn adds error to the final position fix.
As with many technologies, it is easy to be seduced, but less simple to overcome problems unless you really understand a) what levels of accuracy you need and can get by with, and b) what you can afford to purchase.
As always, define the problem first, then match needs with reality.
