At last we've caught up with what the Canadians were all wearing 5 years ago! Various retailers have started importing a variety of high-calf paddling boots, but made from waterproof neoprene. With Easter coming up, I bought a pair of these in 2006 (£60) and having tested them on the Solway thanks to an unexpected swim, I can report that they are comfy, warm and waterproof!
They are significantly more flexible than a traditional welly boot, and a lot more comfortable to wear in the boat as well as being warmer. Although they have a sole that is much thicker than a traditional wet-bootie, they aren't quite as thick as an ordinary welly boot but are still ok for walking over rocks and the like for a shortish distance. Whether you'd want to walk any significant distance in them is another matter as they aren't especially supportive. Far better than wet-booties though.
The velcro strap across the top of your foot holds them nice and snug, and the adjustable calf strap clinches up tight enough to stop water flooding in if you're in water above the top of the boots. Some water will get in though.
Overall, I like them. Quite whether they are a substitute for a well fitting pair of ordinary wellies and a pair of warm socks is something you must decide for yourself. They are very good to wear with a dry suit though. Sizing can be a bit hit 'n miss so you're better to try them on rather than rely on your normal shoe size.
Rapid Kayaks are selling the latest version for £84.95 (as at Nov 2011) under the name "Boundary Shoe".
If you've got big feet and have trouble getting into a boat while wearing wellies, do check that you'll be able to get in wearing these as the sole is fairly chunky. By way of example, my size 11's will slot into my Quest if I'm wearing Hunter wellies which have fairly small heels. I can still get in wearing the Mukluks - just.
I also use Hunter Wellingtons as paddling footwear - not cheap in comparison to a pair of gardening wellies from your local DIY store, but they fit much better (less water space inside!) and last forever.
They have a small(ish) heel which helps if space in the boat is tight and are supportive enough to walk a reasonable distance in. The sole is good and they are exceptionally comfy for what they are. Cut the metal cinch strap off unless you need it - it tends to catch on cockpit rims and doesn't do them much good.
RRP for Hunters is around £60 and you should get them for much less. They are also good for gardening - I'm told.
(Update - June 2013 - Sadly, since Hunter moved production from Scotland to India, I'm hearing reports of a massive decline in quality - with boots lasting only 18 months or so. See this discussion)
Mike Buckley, April 2006 - updated June 2013