sigma 150-500 lens

Hi all i am after some more advice if you could, i have recently found it very frustrating taking bird shots with my sigma 300m lens so i am thinking of buying the sigma 150-500mm lens which i have seen used by a lot of birders how good is this lens and will it be manual or auto with my k-x,i need to know before i think about shelling out £600 for a lens which i think should be good but youre opinions would be great thank you .brian ps, i understand we can also get a discount for being on this site from sms is that correct!!

thank you both for youre comments its made my mind up now ,all i have to do now is find the money.Great photos in youre sites and galleries by the way,thank you both again .brian