They're fine so long as you aren't really expecting to take insect photos. You need to get soooo close with them, and as most small creatures move erratically/get spooked when you're a few inches away, you'll get blurry subjects.
Buy the 10 quid non af ones on ebay and see what you think before shelling £50+ on the AF version. My competition shot in the current 'Springtime' comp was taken using £10 tubes and the 50mm f/1.8.
Note also that most shooting will require a really small aperture ~f/11 or smaller so you'll either have to bump up the ISO or use a tripod to counter slow shutter speeds.