It's easy to get 'templed out' in Sri Lanka, but I wouldn't recommend recommend missing this one.
This place is a marvellous collection of ancient ruins. Walking around it's really easy to imagine how the place might have looked nearly 1000 years ago. The sites are scattered so you'll need to hire either a tuktuk or bicycle to explore it fully if you don't have a driver.
The museum at the start gives some detail about the history of the place, however I would have liked to know more about each site as I was walking around it and information boards are lacking - so either take a decent guide book with you or splash out and hire a tour guide. I wish we had.
Also for those complaining about the $30 fee for tourists...get over it. It's a pittance and besides, it is the Sri Lankan's national treasure to be preserved and not exploited by hoards of foreign tourists.