Humm...I've found Sigma lenses vary quite a bit in quality, but its often a case of doing a little homework before you buy (there are plenty of lens reviews on the internet).
For example, Sigma's 300mm F2.8 is really very good when you drop a UV/IR filter in its filter bay.
http://www.atscope.com.au/widefield/m16m17hXp.jpg
Their 70-200 F2.8 zoom is also excellent and rivals Canon's L series glass in the same FL.