I used to own a gambler - in my pub.
There are many myths surrounding gamblers here's a few:
'I bought a key off e-bay which enables me to win every time' - er no its just a re-fill/volume control key. The machine will only pay out when it feels like it - they are all fixed. People winning all the time - yes it is possible - just, but only if you find a machine that is new and discover a 'back door' feature -this is usually rectified by the manufacturer within the first 12 months.
How much does the operator make from a gambler - un-printable it's that obscene. (but then again the price of the licence is obscene too)
Old gamblers around 3-4 years old are available on e-bay for around £150
We've had machines in our pub that with a bit of common sense would see a positive payout most of the time, at least with the staff who knew how to use them. It's about knowing when to stop.
We let the dumb customers fill them up and we'd empty them after work with our tips. If the right machines were in I'd win beer money and double my tips.
As for those keys, they show how full the hopper is. The fuller the hopper is the longer it is since someone has won big, so your chances are technically higher.
The cost of a license is about £245 a year isn't it? I remember my pub paying about £700 for three machines.