I was approached by them a while ago. They are launching an Irish version of the site, which they say will be getting high volumes of traffic.
Personally I do not think it would be worth it due to the annual fee AND commission on every sale made throught them. If my memory serves me correctly, the annual fee is more than enough to pay for a listing in the UK Yahoo! directory with change to spare.
The annual fee is just under £1000. - didn't seem to be negotiable. And of course you get the commission too so it's not cheap - depending on your product, it can take a lot of sales to justify the annual fee.
In other areas of marketing, you at least have a feel for your ROI and can call a halt if it doesn't work out but this one seems a lot of money for such an unknown.
I've been with them for a couple of years now. Sales are patchy and to be honest I doubt I'll renew when my annual fee is due in December. Sales were decent enough last year, but this year I haven't done anywhere near the same amount of sales, in fact I haven't had a sale in months now. Very disappointed. I get the impression they only care about the big-money merchants these days and just leave us little guys to fend for ourselves, I've had no marketing input from them or any attempt to promote my products at all recently. Really can't be bothered with them anymore.