Doneitall - just to balance out your glowing report, I can write exactly the same about Reigate Grammar! My darling neice and nephew both go to RGS and it is a fantastic school. My brother/SIL actively chose RGS over Caterham (nephew had scholarship too) and are convinced they made the right choice.
The school has seen lots of new development in recent years. It has the most amazing music department, and in the last couple of years a new indoor swimming pool and a state of the art new science block.
RGS just had a recent ISI inspection which rated it Outstanding in many aspects (in fact the report is apparently almost identical to Caterham's from 2010) See here
The current head from RGS is retiring at the end of this year, and my SIL says the general feeling is that whoever takes over is likely to move the school on even further in the future.
I've been to a number of events at the school and the atmosphere at e.g. the drama productions is akin to a West End production - they are fantastic.
Caterham makes an awful lot of noise about it's 'thinking styles' way of teaching (a la Edward deBono...) but a lot of it is a bit of a marketing gimmick. Awards are all very well, but I think a lot of parents would rather the Head & teachers spent time focused on the children, rather than traipsing round the country making presentations to the HMC!
I think the two schools are very similar in terms of results, and where they are ranked depends on what you are comparing. Caterham does very well in A/AS levels due to the high proportion of foreign students coming into the school just for Sixth Form. RGS's GCSE results are excellent, and the league tables don't reflect that the school follows the somewhat harder iGCSE syllabus.
The recent FT school league tables rank Reigate Grammar at 90th in the country and Caterham at 96th, so they really are very similar!
My SIL says RGS has worked very well for her kids because they are at the heart of the community, can walk to school, and in the holidays and at weekends they have all their friends relatively local. One of my nephews best friends went to Caterham and now his friends are spread over a 20 mile radius, which makes his social life harder (more Facebook & less face-to-face!) and his parents permanent taxi drivers!
Anyway, as any parent knows, it's horses for courses, and it seems both these schools offer an outstanding all-round education...