I have also just received this today, although the letter states that I have 28 days from receiving the letter to reply.
I'm not sure I see many downsides - the 6 month notice to leave changes from age 50 to 30 years service (so 51 if you joined at 21) but that's about it. I realise that the pension issue for those that were within 10 years of 55 on 1 Apr 12 (I think it was 12?) will have a significant impact on some, but not for me.
If anyone hasn't run the scenarios through the pension calculator, do it now! As a mid 40s PAS Sqn Ldr on AFPS75 (soon to be 15), I can leave on my 55th birthday in 10 years or so, and get about 100k lump sum plus just over 29k /yr. If I do 5 more years I get a 69k lump sum and just over 46k /yr. 17k /yr extra for a loss of 31k lump sum is a big boost even taking into account that the lump sum is tax free and the income will be taxed at 20/40%.
I see that (20x46k)+69k is 989k so below the magic 1 million mark!
If anyone has seen any other downsides that I have missed I would appreciate it if you posted before I sign the form!
EDIT: Agree about the increments, though. They need to add 5 more, perhaps only accessible if you sign on to 60, but I don't see it happening.