I've googled for this and haven't found much, so I thought I'd ask for people's experiences.
I'm happy now unit testing my models, views and controllers using RSpec. But rspec_on_rails doesn't have any functional ("integration") tests. They recommend using either Watir or Selenium. I've had a quick look at both.
What really impressed me was this: http://blog.aslakhellesoy.com/articles/2006/12/02/getting-screenshots-from-watir . It's interesting that he's got it working on a mac (I thought Watir was windows only until I found out about safariwatir). We have two macs at work (ironically, neither of them used by the designers) and I want to test cross-platform compatibility.
I've noticed this post: http://rubyforge.org/pipermail/rspec-users/2006-September/000049.html too, so it looks like I can have the functionality of either wrapped in RSpec.
Which leaves me with the dilemma, which one do I use? I'm too pressed for time to try them both extensively, so I was hoping someone here could give me a rundown of them (I've found http://narayanraman.blogspot.com/2006/03/sahi-vs-selenium-vs-watir.html but I assume it's already out of date.) From a quick look over, Selenium looks more powerful, and Watir looks more accessible, something you could train a QA team to use without needing a developer (our developers ARE the QA team, however).
I'd love to hear some more informed opinions so I can recommend that we use one or the other to my boss. He's already extremely impressed with the screencasts of both.