Like all classified ad sites, and especially the free ones, Gumtree does indeed suffer from scam ads. That said, they seem pretty good at taking down scam ads when reported and there are active forum moderators who aid in that cause.
The site itself carries a growing selection of ads and it's free. I think the site is a good choice for items that you can buy locally and transact face to face (which was, I beleive, the original concept behind the site). But don't use it to buy things remotely from people you haven't met.