I've used Selenium. The features were good, and it was usable but it was buggy.
The IDE would often record events incorrectly (so tests would need to be manually changed), and test files occasionally became completely unusable for no apparent reason, which meant they would have to be recreated all over again. Also development on Selenium IDE seems to have stopped; there hasn't been any bug fixes and patches for a while, and bug reports seem to go unnoticed.
Molybdenum is an alternative, built on Selenium that's worth looking into.
http://www.molyb.org/