Hi!
I was in Gambia in September 2010 (at the end of the rainy season), and without doubt it is an absolutely recommendable trip.
The country is very beautiful and, as it is small enough, most of the interesting points (all the beaches on the south coast, natural parks as Abuko or Makasuto, fishermen villages as Tanji) as easily accesible from the resorts. Of course, if you have time it is wonderful to do a visit upcountry (Tendaba, maybe George town) to see real Africa, small villages, wonderful riverside landscapes...
I would like also to recommend our taxi driver there, Yaya. He was our guide for several days, and it is a very nice and reliable man. He has a good car, is a very nice person, and as a guide he'll offer to you wonderful places to visit. He only speaks English. His mobile is +220 983 9885. I have no special interest about promoting him, just that I think is a good man and also a good driver and guide.
Just only one thought, many people try not to visit Gambia during the rainy season. I think that at the end of the rainy season the landscapes (much more green) are really pretty!
Enjoy Gambia!
David