I've flown into a fair number of military fields without any problem and agree with the advice above. If you are flying IFR you may well be offered a PAR approach (like an SRA but with glideslope guidance).
There some fields with an RAF or RN museum on site, which makes for a really interesting day out. You may well be offered ground transport over to the museum.
When you ring for PPR, you may be asked to fax a copy of your insurance to confirm MoD cover. Some aifields will also ask you to fax details of yourself/passengers and aircraft, presumably for security reasons.
I have had one case where the airfield did not want me to pay on the spot and insisted in invoicing the charges.
So it's a little bit more hassle, but worth it for a new dimension in flying experience. However, I should say that all my experience was before we were at war.
AA.