I absolutely agree that typically the impact you can have at a big company can be huge. But it may come with a cost - adjustment to that structure.
As I mentioned in the post, it is a great environment for many people, and I'm glad to hear about your case.
I've also found that large companies (or at least Intuit) are very open to progression and innovation. When I started our team was on Ruby 1.8.7, Rails 2.3.5, in a physical data center, used Perforce, and a bunch of other "enteprise" software suites. Now we are cranking on latest versions of Ruby and Rails, Github, AWS, and Campfire.
It has been a much more rewarding experience than I would have thought. Oh and you have no idea how good it feels to get real, publicly traded stock :)