I have been trying large uncooked shell- on prawns for the last couple of months. To be honest all that I have managed to catch are woofers and a small Turbot, so not the best of bait as far as i'm concerned.
My dad used them in Ireland last month. The only thing I have caught on them is doggies and whiting but on a couple of occasions when it was a bit slow he was pulling out flounder on them when the lug/rag wasn't working. Unfortunatly we ran out before I could try them.
I'm deffinately trying them this winter.
Another bait worth trying is our native prawns just thread them onto a size 4 or 2 hook head first I've caught dabs, pollock coalfish, whiting, bass (v small) and they'll probably catch more species, they don't have to be live and kicking.
I've been using the Tiger prawns (headless) from my local chinese wholesalers at a cost of £6.99 for a 2 kilo box. They have fished well out of North Wales taking plaice dab and gurnards but don't seem to work on the north east coast at all. (advantage is keep em cool take em home and have em for your supper!)