We've done murder mysteries for our New Year's Eve party the last few years. They have all gone over great! People love the interactive entertainment aspect. We've used the same company over the past couple years and will use her again this year because she just came out with a sequel to the first mystery we hosted:
http://www.dinnerandamurder.com/
The design of the mysteries is great - gives enough direction so that people don't feel like they're floundering, but it's not scripted. And, it's easy to incorporate as many people as you'd like, and everyone has a fully integrated part. She is currently working on a supers (werewolves, vamps, mages, etc) themed murder mystery, which I'm not sure will be out in time for Halloween. In addition, she has links to a couple of other murder mystery sites in case none of her mysteries grab you.
I've seen the Ace site and thought it looked worth trying, but since we only do one a year - we just haven't gotten to them yet! :-)
And after you look at those, here's another if you're interested:
http://www.murder-mystery-party.com/
Final word is that we have had great success hosting murder mysteries - the guests (and us) have a wonderful time. I highly suggest doing it!