As far as I know the site does get a fair amount of traffic, spread over its numerous specialised categories/domains. Each category within the directory appears to have an equivalent independent domain which allows the directory to be listed in detail across the web, rather than every link just pointing to the home page. Thus a certain amount of traffic is assured in this way.
It is on the pricey side in comparison to other UK directories, given that you can get listed in BT Looksmart for £175 or Yahoo UK for £199. The only one that I know of that is more expensive is Just35.com, which charges £1820 per year to be listed.