I have recently been working with another developer that has been developing on his Windows laptop. He's been using the same tools and Rails 3 codebase as myself (I'm on Mac OS X).
He has had a number of blocking issues that have wasted many hours of his time - for example he hasn't been able to run any of our RSpec tests, and some Rake tasks.
Getting the environment setup is a pain, and even when you do, it's stressful because you never know when you are going to hit the next bizarro Windows-only bug. The reality is that Windows is a rare OS among Rubyists, and even if that wasn't so, the Rails framework developers themselves are UNIX-oriented, so there is an order of magnitude less scrutiny given to testing and development on the platform.
What's more, many of the sweet tools in the Ruby world are command-line and they work better in a real POSIX shell.
My co-worker has now bought a MacBook, and is enjoying a smoother development experience again.
But you don't need to buy a Mac; You would do just as well with dual-booting into Ubuntu (Or similar) on your existing laptop.