It is going to depend on what you are using them for.
I have been using GPS devices for over 10 years now. My favorite all-around device was my Magellan Meridian Platinum. I have a Magellan Maestro 4250 in my car. The Meridian I used for sailing, hiking and geocaching. Still probably one of the most accurate out there.
I have had an in-car GPS for a few years already and it has never failed on me. It already has a mount and in-car power. For driving my car, I would stick with it. However it is not because I think the maps/gps would be worse with the iPad - I just don't want to have to find some crazy mounting device for the iPad.
As for the iPad, you get a nice, big, distracting screen. It really isn't a good size to sit on your dash unless you have a huge and high windshield, so you would be looking off the road to see the maps. For a one driver car, I would not recommend using the iPad simply for that reason. HOWEVER - the iPad will always have the most up to date maps available online, live with 3G. I would not recommend using the wifi-only iPad for anything but overview and "general area." A jailbroken iPad with the BT stack open, you can use a BT GPS puck.
Using the iPad as a door-to-door unit will work great with a copilot. I can also see uses for it for boating (get a good waterproof housing and solid mount) since looking down for short/medium time periods is not as critical as it is in a car.
For Geocaching - MotionX GPS is awesome. I recommend a good, solid case which can protect the iPad in case of drops and inclement weather and such. Does it do better than my Meridian? No, however being able to view cache details and the website while on the trail and not have to have a separate device or carry printouts is a definite plus. I definitely recommend it for Geocaching.
It really comes down to what you want to use it for and what you are willing to do to make the iPad a good GPS unit.
iPad Pros:
1. large, easy to read screen
2. always up to date maps - MotionX products allow you to download maps before getting on the road so you don't use 3G or have to worry about wifi.
iPad Cons:
1. large screen - not great for cars, trucks maybe. Special separate mount required either way.
2. not rugged without spending money on special cases.
A little extra planning and care is required for the iPad, but that is not a show stopper. Like I said, I use mine for door-to-door as the navigator and I use it for Geocaching and love it, especially with the 3G being able to update cache findings live.