I've got the Garmin III Pilot. I love it, and would definitely recommend it. Its nice and small, and its got everything you need- runways, freqs., airspace, moving map, etc. The backlighting also works really well (but if you're going to use it a lot at night, get a cig. lighter adapter). You can also download software and database updates right off Garmin's website, which is nice- no waiting for chips or disks. With the exception of com/nav and approach capability, it can do anything a Garmin 430 can. Mine is mainly a backup now, but our three primary trainers don't have GPS, so I use it quite a bit in them.
The 152 that I rent has no GPS and after flying in a 172 with one a few days ago, I thought that it would be a great backup for the standard pilotage and dead reckoning. Are there any accessories that are essential when purchasing the GPS.
Might want to look into the Lowrance. If I can get the cash together to get a GPS it'll be that one (inless I win the lottery in which case I'm getting a Garmin 196). It comes with all the extra stuff standard that you normally have to buy from Garmin. I hear it's the man, but only what I hear.
I "second" everything PA44 said. I would add the the RAM Mount yoke mount kit for it is very nice to have. I put a RAM Mount suction cup on the windshield of the truck for the road trips. The RAM Mount products were purchased at Aircraft Spruce and Specialty Co. as they were the cheapest place I found at the time.