To be fair, basgetti, modern OFSTEDs are all but meaningless, aren't they, though?
To decide on what's a good school first you need to decided 'what's good for my DC' and work on from there. A high achieving, fast moving academic school with great GCSEs might not suit all DCs (but until recently, would have got a good OFSTED even with a low 'value adding' score); now, VA seems to be all the buzz word. And I say hats off to a school that has an 'E' grade achieving intake and gets Ds and Cs out of them. Well done. But if your DC is capable of an A, they may not do so well is a school geared to the borderline DC, iyswim; they might be better off at a school that takes B and A grade students and keeps them there, the VA will be low but your DC will have the A grades they need.
My DC go to a high achieving school. DS1 is doing OK there, but I chose the school for DS2, really, as I saw him as being less academic but was worried about how he'd go in a comp where the lower sets not only have the lower ability DC but also the clever-enough but can't be arsed as well as the anti-social, wrecking the lesson for everyone else. However, I have to say that DS2 is also doing very well academically at this chosen school! I bet the school's VA is rubbish thus its next OFSTED may not be as stellar as those preceding it but it's working for my DCs.
As for good schools going downhill and vice versa- well, I shall chuck some controversy in here and say it all depends on whether the 'greatness' of a school is dependent on one or two people dragging all else along. An example is a well-paid, flown-in Super-Head appointed very publicly into a brand new Academy, complete with Billy Bunter uniform, built to replace a couple of dire comps in a deprived, challenging area who within a year can show an amazing turn around in results, albeit not at GCSE at that takes, imho, at least 4 years if not the full 5 to achieve. 2 or 3 years later, he leaves, the lustre has worn off, the money tap is running dry and once again, the school is just left struggling with difficult, non-school ready, deprived DC to try and educate; whereas the leafy comp in the leafy MC area can almost run like Belgium 
who went was it 20 months without a government? I am being tongue in cheek when I say that, but our Head need be much less 'dynamic' than that Super-Head. Of course, I know our Head actually works his socks off (and yes, works 'the system') to keep his school where it is at the front of the league tables but the fact remains that, in some areas, a good school usually remains that, one way or another, if catchments don't change or demographics shift too radically.