Not knowing what branch you'd be going for, I can't make specific comments. Seadog has covered the basics, but I'll try to put a slant on it. Ex-RN are pretty common throughout the RFA, especially in the Communications branch. On the last ship I sailed on, we had an AB who was an ex-Chief Mine Clearance Diver, and our LH(HC) was an ex CPO(seamen spec), both of whom thoroughly enjoyed the job.
The leave and terms and conditions are excellent - equating roughly to 4 on 2 off, with single cabins for all personnel (though generally sharing a head and shower with the cabin next door, or shared washing/showering facilities). Rather than messes with a mess square, there is a crew bar. This is a large bar, with TV, sound system and other entertainments (generally including a dart board), shared by all the Junior Ranks on board. The pay as a complement SG1A (equivalent of AB, after about 18 months in) is somewhere around £25k per year, with opportunities for promotion if you're so minded. The rank/experience structure is very different, with a massive age range amongst the ABs.
Downsides - Portland, Loch Striven, Glen Mallen, the buoy in Plymouth sound. Alongside berths are not brilliant and tend to be very much in the middle of nowhere - though contrary to rumours we don't spend much time at anchor. RFAs do spend a fair amount of time on the buoy in the sound, but when they are a liberty boat routine runs out of millbay (though the midnight last boat is a bit of a pain). Although you work 4 month trips with 2 months leave, you work all those 4 months - 7 days per week (half days at the weekends), and when alongside you don't automatically get the weekend off if you aren't duty. If alongside, you will probably get 1 weekend off in 3 or 4 (midday friday to midday monday). If you're at sea for long periods (up to 70 days between ports), the routine can get rather mind-numbing. As seadog says, there are a lot of the older die-hard merchant seamen who have a very blinkered outlook toward the RN (this includes some officers), but they tend to be anti-RN in a general sense rather than anti-ex RN personnel.
Any further specific questions, feel free to PM me.