My wife uses a couple of Peacock handwarmers of similar design to the review product for our night-time/winter sea angling sessions. They've been on the go for a good 6 years and work as well today as they did when new.
I note that the Peacocks currently sell for £25.95 + £2.95 p&p, so if the Whitby model is of similar size and heat output, then it is definitely a good buy.
THe only note of caution I would add is that the burner units will eventually burn out and require replacement - the time-frame is dependent on frequency of use, of course. I see that Whitby sell replacement burners, but I couldn't find a price on their website. The standard Peacock replacement burners currently sell for £9.50 + £1.50 p&p which, if replaced regularly, represent a significant additional cost to ownership.
Still, my wife swears by them so they are a "must-have"! And she can go one better than burning her leg, as mentioned by Richard above. In the interests of decency, I won't mention the affected body part, but the injury could only be ameliorated by compression with 2 frozen mackerel brought along for bait!!!
Keith