Garmin is very accurate in terms of speed as with just about any WASS Enabled System
Garmin is very easy to use etc like everyone said.
For me, the single two biggest attractions to Garmin are the constant and fairly consistent updating of info and the ease of which to aquire it.
They release updates to the mapping software. You can choose which software you want. They also stay on top of bugs and such within the unit and offer lots of firmware upgrades which help with efficiency on how the mapping software is used etc...
For performance boats where there is seldom unlimited space, I like the units that have their own internal "lithium" battery. If you hardwire the unit to the helm, it constantly maintains the health of the battery. So----
If you have a breakdown where your boat's batteries become discharged, there is a backup power source for your GPS so you know where the heck you are and you now have an extened time for which to use the GPS itself...
If you have a DSC enabled VHF and you link it to the DSC output on the Garmin, it will broadcast your exact coordinates when the mic is keyed on 16.
The catch is that your service provider has to be setup to receive DSC. Worth looking into. It's a way nice feature. Roamless private calls with DSC and really makes it nice for someone who is looking for you on a call for help.
BTW, the VHF phone calls, the availability for someone at home to view your location on a pc and "DSC all inclusive" is the "SeaSmart" offering from Sea Tow. I'm currently not familiar with another way to do it.
Sorry I rambled.