The Sigma AF 55-200mm f/4-5.6 DC is a budget lens
and it shows in some aspects such as small max. aperture combined with a limited zoom range
and some mechanical shortcomings (rotating front element and coupled focus ring).
Nonetheless the lens is also capable to deliver quite impressive resolution results.
So if you can live with its shortcomings and you just can't spend more for a faster or longer tele zoom
the Sigma is a viable option.