can't really comment mate - racking my addled brain but can't recall ever having used it! (in my defence I usually only stay one night... two at most...)
My favourite is http://gwynant.com
Maybe not the most convenient for climbing but has good facilities, is a nice quiet site and well run. You can even get a freshly cooked pizza some evenings
My current favourite site.
New shower block , fire pits and wood available . Overlooking Llynnau Mymbyr
5 Quid a night .
You could probably walk to the pub in Capel Curig from it.
The one opposite vaynol is cheap, £5? But has pay showers and no bog roll. Ideal location but I'd consider somewhere fancier if more than 1 or 2 nights