I've stopped using wood pellets because although I found it hid the smell beautifully, I was getting utterly fed up of the mess! One of mine has such a long tail it would get covered in sawdust and it got all in their feet too so they'd tramp it everywhere. Literally everywhere. So unhygienic. I just found a really really amazing one at Pets at Home, also wood based but clumping. it's called Cat's Best Öko Plus, and it is really brilliant, clumps so well so the pee can be scooped out right away, it's natural and compostable wood and it doesn't cost as much as the world's best. It's about £5.99 for 10l, but cheaper online, and since you don't have to change it as often since you remove everything when you scoop, it's really economical.