To see the Lago de Atitlan, Panajachel is a must-go; however, I found the city itself utterly forgettable. The lake, however, is a must-see; possibly one of the most beautiful lakes I have seen, save for the 4000m high lakes in Tibet. The lake is ringed by volcanoes and has a beautiful blue tinge (although whether raw sewage contributes to this or not, I am not sure). While there, we did not go to the other towns surrounding the lake, as my friend suggested that they were forgettable; the main reason to go to the lake is to see the lake itself. However, water taxis were offering 25Q p/p one way to the other towns. Apparently, there is a very difficult-to-get 10Q local price, but we were not able to secure it. There is also a public boat that goes on the coastline of the lake from the Panajachel dock which costs 10Q p/p and is worthwhile, if only to see the beautiful houses of the Guatemalan elite. These houses are built into the mountains and accessible only by boat.
Otherwise, it is possible to swim in the lake as my host father attempted, although the water is pretty frigid and of questionable quality. There are food stands along the road bordering the cost (fries and chicken for 15Q), and enough restaurants to satisfy any need. The road to Panajachel was undergoing construction as of Jan. 2009, so book more than the allotted 2 1/2 hour time from Antigua. A minivan with twelve people inside (my host family + guests) is also an interesting, alternative way to travel!
Note: pictures courtesy D.A.