I just wanted some opinions from people that have run Selenium (http://selenium.openqa.org) I have had a lot of experience with WaTiN and even wrote a recording suite for it. I
Selenium is pretty decent tool but there are couple things to watch out:
Selenium IDE and Selenium core do not share 100% same functionality. For example right clicking is supported by IDE but current core release does not have it. However, using a newer version from their repository solves that.
In case of ext js, gwt etc make sure you have proper IDs for your display elements instead of automatically generated (random) ones.
Maintaining test cases. I have seen cases where a lot of effort was put on Selenium tests and good coverage. Later on tests started to fail as the person creating them was busy with other tasks and no-one else wanted to touch them. But this was issue with management, not Selenium.