Been lots of times (10+) in a C172, never once filed a FP however I always phoned for PPR & a 'chat' prior to dispatch, very friendly helpful bunch at Scillies ATC.
It is still one of my fravorite places to fly in & it is must to fly around to have a good look at the islands while you are there, check the availability of AVGAS too as I am sure it is pretty limited if none at all, not sure these days!
It is more than long enough but some caution is required due to gradient, different sufaces e.t.c!
Enjoy
