I didn't have a WereBear when I was small, though I did have a WereBear comic and the song from the advert remains stuck in my brain.
As a regular teddy bear, the WereBears are - as you might expect - exactly the same as a regular teddy. Soft and cuddly with nothing to indicate the horrors beneath. Turn the head inside out, do the same with the paws and you have a really quite fearsome beast. Evil eyes, claws and plenty of long, pointy teeth (except for 'Gums' who came toothless. It's no surprise he was the least popular with boys). The teeth are plastic, but soft and thus WereBear can still be cuddled without fear of injury.
One thing I noticed, having one in hand after only 30 years of waiting, is that WereBears are machine washable (the first thing you look for as a parent). No 'surface wash only' with these guys. No that I've actually attempted to shove it in the washing machine, but the option is there, apparently.
Originally WereBears came with a cassette or 'tape' (remember those? Children ask your parents) with a background story about the bears. These are much harder to come across these days than the bears themselves, but luckily some thoughtful person has put them all on the internet for download.
The stories, plus a load of background information, are available here.
Wikipedia tells me that there was a relaunch of the bears in late 2012, with them being available (of all places) in Farmfoods. There's also a shiny new(ish) website and a fancy CGI trailer. Beware when watching this trailer, it will make even the bravest of people (notice I say people not just children) jump in fright. To be honest, I think it's a little too over the top for the intended child-audience but here we go...
4 bears were initially released, back in the day, followed by the larger Growler (with roaring function) and the 4 TerrorTeds. From my (limited) research, the standard bears are reasonably common on ebay and the TerrorTeds not so much. There are two things to watch out for: 1) Check that Gums and Fang have their hats, and 2) Ensure you're certain whether you're getting the 80s versions or the newer ones. Auctions that start their bidding at 99p tend to not go for very much, but there are plenty of sellers starting the auctions much higher (and thus having their bears remain unsold).