I was there a while back, based in the exciting and exotic city of Stansted. My impressions of the company are as follows:
-if you are single and unattached, Atlas is a great gig. Lots of variety of flying. Great hotels, equipment, and crews.
-Gateway basing seemed like a great deal (to someone on the outside looking in). Being based in Stansted, I didn't have it, which made the job quite a bit different for me than those with gateway basing. I think they've resolved this issue now, but someone with more recent experience will have to chime in.
-The 50 hour probationary guarantee is a huge bite. I took a major pay hit coming from left seat at a regional. I never once broke 50 hours my first year.
-Part of the beauty and curse of ACMI flying is the unpredictability. It's just the nature of the business. If you have a major problem with your schedule being completely re-written in the middle of a trip, this isn't the kind of lifestyle for you.
-I turned in my resignation after having two 17 day blocks back up to each other, then getting extended on top of that. Again, if I didn't have any pressing reasons to be home, it wouldn't have been an issue. If you have a family, particularly a young one, this puts a lot of strain on it.
Good luck