I was going to send loads of DVDs and CDs to them until I entered the barcode and saw that all I'd get was 30p. I'm now toying with the idea of just selling them incividually on ebay for 99p or whatever the final auction price is.
I have used it- it is convenient but won't make you rich. Thye always paid up quickly- you enter the barcodes, they give you a price, they post an envelope, you post them off.
If it's anything like the jewellery sites, I wouldn't bother, they are banking on people not taking the time to sell on eBay etc. Those jewellery sites got a hammering on one of the consumer programmes on TV last week.