I'm not ex Mil but I know plenty of folk who are and the things I'm most often told are by those who make the switch are:
1) In some cases, how much harder than expected, they found the civillian courses and find the subsequent transition at the unit equally difficult. There's a big difference between certain aspects of the Mil and civil atc job.
2) Once they've got a CAA licence and a job, being an ATCO is the only thing they have to do, (they're no longer required to do secondary duties), there is no pressure to go into any sort of management unless you want to.
3) The pay structure is significantly better.
4) They won't be moving house every three years or so to satisfy the needs of the MOD.
5) A very small number take a long time to get used to the fact they're no longer an officer, they're the bottom of the heap for a while and no one will call them Sir, but it generally is a small number.
On the whole, most ex Mil atcos who have become civvies are cool dudes and a pleasure to work with, IMHO, a lot of that is to do with the fact that they've had to work twice as hard to get where they are.