I haven't used them but it is similar to CEX - the price varies from CD to CD depending on demand and supply. If they are only offering you 30p each for the CDs you are trying to sell then they are titles for which there is very little demand.
Think I sold 98 CD's to em and got about 40quid. Certainly not a great amount, but what's the alternative? Get even less for them off Ebay? Don't think even charity shops will take them now, and you'd be giving them away to them.
I was offered £2.28 for what i considered a fairly rare DVD, decided to take a punt on Ebay and got £21.25 plus postage, bit of a no brainer really. I would do what a previous poster said sort through your collections, if you think any of them are a bit out of the ordinary take a chance on ebay, likelyhood you will make more than you would of these rip off merchants. Any run of the mill stuff is probably worth lumping together and selling as a job lot, don't expect much though.