We use it quite a lot in our company, but that is mostly due to the fact that we had .NET already for other applications. We didn't have any automated testing or stress testing at all until I looked at ACT (since we already had it). Now that there is a need for that sort of testing, we've stuck with it, and they're unwilling to invest in a more robust application because we can get ACT to do anything that they need (with a lot of scripting). We've used ACT against all of our production environments to good success, for both automated stress testing and automated regression testing, but none of those tests were at all useful immediately after recording, they had to have a lot of script modifications made...which, of course, requires more time.
Eventually I'm hoping to at least get time to evaluate OpenSTA as an alternative, as it seemed to haev some features ACT lacked, and it's free.
I prefer 'maliciously mischevious'.
TDAdmin of Evil