I expect it will behave differently at every company as they input their own parameters into it. That being said, I was less than thrilled with the first few months that we had it. However after that I've been generally pleased with it. I believe that is a combination of the company changing some of their parameters and myself learning some of the logic behind the system so you know what to bid. It's not perfect, no system is. It doesn't have the human touch like when a person is doing the building but I think we are getting to the size where it would be very difficult to hand build schedules.
As I said there are problems with it but of the people I talked to who are unhappy with it a lot are bidding in such a way to gaurantee that they will be unsastified. Don't get too carried away with request, ie. late checkin, early checkout, 4 days off between pairings with overnights in Cancun and stops in St. John's. If you try stuff like that the items you request may be mutually exclusive. I find if I ask for the days off I need and then a few simple requests beyond that it generally works well. This month I had no particular days off so I bid 4 between pairings with late checkin and early checkout and I'm very happy.
Hope this helps