Its a terrific lens.
The speed and lack of VR/IS obviously limit it slightly, but thats why its "only" £700.
Don't be fooled into thinking this lens is any kind of compromise on image quality just because it hasn't got Cankon written on the lens cap.
It loses a little bit of bite at full reach, just like the 100-400 from Canon, and the 80-400 from Nikon both do,but don't forget it has an extra 500mm to play with.Just zoom back 100mm from full reach and you're at 400mm and pin sharp.
Having said that, even at 500mm the lens is more than capable of producing excellent results.
EDIT: BTW, don't forget get shutter speeds when using this lens.Its big, heavy and long.The slightest bit of camera movement is amplified 10 fold.