Got the APO version of the lens and I have no qualms about it at all, for the price I think it is fantastic, I use it for most things be it birds, wildlife, candids or more recently studio shots for my college work. Most of the stuff in my Flickr is taken with it if you are looking for some samples.
If you are prepared to pay a wee bit more though a lot of folk rave about the Canon 55-250mm as it just as sharp and has built in image stabilisation.
Tommy.