Skatingsailor, most of the "OW safe" caverns are fun and good to dive, but you have to accept that little extra element of risk of not being able to get out as quickly (ie no direct ascent), there is also the silting problem. Whilst i havent yet been to Blue Grotto (its on the list), i understand it gets pretty busy and with all that activity comes silting - not good to be caught in the middle of. Whist there may be no problems on the one or two times you have gone there, it is always best to keep the permenant line in view, or just get cavern certified and run your own to and off the main line. I dont profess to be anything but a newbie at this whole game, but i take this overheads seriously when i am there and am getting more training asap.
Forgot to add this last night (in fact computer crashed), but the phrase:
"Its all good fun until someone loses an eye" - is very much the case in overhead environments.