I was so close to buying the Sigma 120-400mm. I even ordered and paid the lens, but I cancelled the order after a fellow pentaxian on a norwegian forum recommended the Pentax DA 55-300mm. It's halve the price, 1/3 of the weight and size and much better!
Just see the pictures on Flickr, there is a 120-400mm and a 55-300mm group pool.
DA 55-300mm don't have SDM/HSM. They say it's noisy, I don't think it's so bad.
There's is one thing Pentax should do with this lens, they should make a focus limter. It would help alot if you could limit the focus range between 10m and the infinite mark when you are "birding".
Some of my first pictures with the Pentax DA 55-300mm f/4-5.8 ED
I bought mine today
The first picture is cropped a bit, from a 14,5Mp to a 5,5Mp picture. And then downsized to 1280*850pix.