I've found it to be quite good, but it's a lot of faff - or, if you do it well, with really good descriptions and decent photos, it's a lot of faff.
I haven't had much trouble at all with dodgy buyers - which actually, considering the number of sellers who say they have, I do find a bit weird. But because I haven't had much trouble, I'd go against the grain and say don't bother sending lower-value things signed for, because it puts the postage cost up and will therefore put buyers off - also, as a buyer, I will sometimes pass on making a purchase because I don't want the hassle of a waiting in to sign. I would put something on as signed-for if it's likely it will go for more than, say, £30.
My top tip would be to try Gumtree first, for some things. If you're going to be taking the photos and writing the descriptions anyway, you might as well. Gumtree is free - you keep the full selling price. Gumtree is all cash on collection, so there isn't the hassle of packing up parcels. And the really good thing about it is that there isn't going to be any annoying comeback - once a things is sold, that's the end of it. I've been surprised by how well I've done with Gumtree. It's not much use for clothes, but it is quite good for furniture and household items.