I've been meaning to rave about Knit Picks Options interchangeable needles, which I've used for my last project and the one I'm working on at the moment.
They are just the best thing going. Extremely versatile and a dream to use. Buying the complete standard set and a pair of each of the largest sizes was worth every penny. Read the hype on the Knit Picks website - you can believe every word of it.
A few reviewers on knitting forums could give you the impression that the metal version of these needles (they now also do a wooden set) are overly sharp and extremely slippery. I don't agree, though you can be guaranteed that your completed stitches will fly off your needles with minimum effort.
Frustratingly, Knit Picks themselves only seem to supply to North America and won't do international delivery. The UK supplier is the Bristol-based store Get Knitted, which gives me an excuse for another rave...
Get Knitted have provided some fantastic customer service. Having had the set for a couple of months I realised that one of the sealed packs supplied by Knit Picks had a mismatched pair of needles. Not only did Get Knitted reply to my e-mail within 24 hours (and on a Saturday too), they were kind enough to send a replacement needle through the post a couple of days later. I like these guys, and good customer service is the best excuse for customer loyalty.
I'd certainly encourage anyone with a use for circular needles to go an investigate this set, and if you need a UK supplier, then head straight for Get Knitted.