First off, call ahead before planning a trip just to go swimming, as when water levels are low (as they may be now, due to our absence of rain in South/Central Texas), they close the area to swimming due to bacteria levels.
Hamilton Pool is a small but beautiful park -- quite lush and green, almost tropical in appearance. Justin and I have hiked there a couple times, although not on all the trails -- some of the trails get closed after heavy rains due to flooding.
For photos from one of our visits to Hamilton Pool Preserve, check out the following gallery/slideshow:
Hamilton Pool Photos (2001)