I've just started looking at Ruby on Rails, on a Windows XP platform, and I can say I'm very impressed so far. You can prototype solutions real quick and get your customer's feedback on what works and what doesn't. Any changes they ask for are trivial to make and so you can get into a real collabaritive situation with them.
The only thing I've got against it so far is that one of it's main mantras is "Don't repeat yourself", but it seems to me that if you want to change the look and feel of your site, you have to repeat that look and feel in all the rhtml files that Rails creates for you. Granted, I'm only half way through the book, so there may be a way around that, and if there is I'll let you know, but for now I'm really impressed. I must also remember to take a look at
Seaside, which is sort of similar but written in my beloved Smalltalk.