We did an appeal (successful) for my older son. My personal view is that it depends on your HT. I found that ours was in the beginning rather focussed on the fact that my son is "nice and excels at sports" and would do well at a GS, rather than he was a level 5 across the board and G&T for maths and needed that particular GS.
If you can get your HT "on message" they would probably be worth taking but they need to know about appeals really and what evidence is useful. There is loads of info on this site, this bit might help though http://www.elevenplusexams.co.uk/appeal ... cation#b48
Katel did his appeal at the same time as mine and his quote in that link was really helpful to me in my discussions with our HT.