Hi Ian,
I paid a visit to Kalematiya in late Feb and it was well worth the visit in my opinion. The location is a large shallow lagoon partially covered with water lily and the species you can expect in such an environment, I guess most of the species are resident so hopefully May is OK as well, but better check. The bird life was impressive, a healthy population.
The road to the place is quite bad and you need a boat for a meaningful visit. I went out for a 3-4 hour morning trip. The hotel recommended a guide, his English knowledge is a bit limited but good enough for this purpose, and he seemed truly interested in birds. The boat was a catamaran built from two canoes, with a large platform, well designed for this purpose, suitable for observation and photography. Propelled by pole so totally silent. I can try to dig it out his phone number if you are interested.
The good thing with the place - there seems to be few visitors. During the 3-4 hour trip we encountered another catamaran with what appeared to be casual birders, and Feb is high tourist season on Sri Lanka. We had bad luck with overcast weather and I would have visited the place a second time, should I have had time which I did not. The guide mentioned that often the place is better in afternoons/evenings.
I'm not sure what you mean with "indifferent", what kind of birding experience thrills you but I personally liked the place.
Tord
