I love the darn thing (Garmin 196)... I don't use it as a nav instrument, but as a general where am I type thing. I do like see the bread crumb feature so that I see where I've been. and the flight logbook is relatively accurate.
Entering waypoints can be a bear (but that's with any Garmin) since there's no keyboard and you're scrolling thru letters and numbers to get a waypoint or airport into the GPS unit.
If you're trying to enter a route while you're in the air, I think personally that you'll be spending too much time heads down trying to enter a flight plan in the GPS. So it's best to get the route into the GPS before you takeoff.
Consequently if you're on an IFR routing, it's a bit troublesome to handle a reroute in mid-air (again because of entering the waypoints thru scrolling -- it's a Garmin thing not a 196 thing) Fly the plane first, then when you get stabilized on the new route, you can decide whether you want to up date route on the Garmin.
Although the 196 (or any GPS) has come in handy because I think ATC expects every aircraft to be able to navigate direct to an intersection these days. Center pretty much tells me to go direct to the destination airport when I'm clear of the departure airspace and traffic (Usually I'm still about 150-200 miles out from the destination and cannot possibly pick up that destinaton VOR). So I ask for a ATC vector and just hit D-> to the dest in the GPS and just monitor where I am. And as soon as I pick up the VOR, I start tracking that inbound.
Bonus animus sit, ab experientia. Quod salvatum fuerit de malis usu venit judicium.