A pda using Pocket Fms is every bit as good and very nearly as reliable as a Garmin however each has a few advantages and disadvantages.
The pda is not the out of the box solution Garmin proivide. It requires selecting the correct pda for your purpose, attaching a gps (be it by bluetooth or cable), installing the software on an sd card and knowing a little about how to use the pda. Potentially these are disadvantages. On the other hand it is far more configurable in terms of both the hardware and software. For example, if you want to run the GPS purely off batteries for three hours or more this is possible with the pda but may not be with some colour dedicated units.
You should read the user forum on PocketFms which will answer many of these questions and give some good pointeres on the best pdas to use.
In short, horse for courses.