It varies hugely. Best bet is to get something in writing from the Unit MO. The guidelines I got were from an RN Surg Capt, but he stressed that it only applied to me, and the job that I was expected to carry out.
I had heard that you would not be allowed to have had it done before you joined the military but it was ok to get it done afterwards. I think it will downgrade you for up to 6months after the treatment to allow your eye to sort its self out, especially important if you are likely to be deployed to Iraq/Afghanistan... lots of dust in air/eye etc.
The MOD really need to have a good look over the different variations of laser eye surgery. I believe the 6 month healing time was in regards to one of the first forms of treatment, which did require a sufficiently long healing time. I have had the LASIK treatment which required a healing time of a fortnight. This was the quickest and most painless procedure, side effects are minimal however a slight halo effect does effect me at night with regards to streetlights etc. I can see why the MOD restrict this with aircrew etc.
I currently work in Baghdad in the private sector and have no problems with dry/dusty environment following the procedure. Working with the septics ive learnt they provide this procedure as a freebie to some members of their armed forces who require it.
The only real problems are immediately following the procedure with people neglecting to take their post surgery eye drops thus resulting in infections etc.
The 6 month downgrade period isn't to do with healing time, but rather to assess whether the procedure has been successful in creating a prolongued improvement. Some laser eye surgery patients report excellent results immediately post op, only to find that further deterioration occurs over the subsequent months. As stated previously, it is wise to discuss this with your MO with a view to assessing the suitability of the procedure within your employment group or perhaps your future aspirations.