I did one to clear our loft of pink stuff as soon as I found out my bump was blue.
It was definitely worth it to get rid of a lot of a lot of stuff in one go but I was pretty surprised by what sold and what didn't. I buy second hand to get better quality stuff but people cleared out my baskets of £2 crappy 'nursery' stuff but left a lot of nice Gap dress/knicker sets for example. At about £4 and in lovely condition that's what I would have been looking for and I managed to get decent prices later on eBay. The other items that went well were fancy dress, coats, Clarks shoes and boots. At the end I met a friend from work's wife who was selling boy stuff and having a girl so we swapped a load of stuff.
I haven't been to any others but this was at the Aspire Centre in Southfields and quieter than I thought it would be.
After paying the fee and buying a rail I probably made about £120 but it was the cleansing feeling that made it worthwhile