Yes it is, but it depens on too many factors to describe here. An important one is how steady you are yourself (I can shoot 200mm at 1/80-1/100 without difficulties). If you are not IS (Image Stabilization) can prove of great help.
EOS400d + Sigma 18-50 + EF 24-70 F2.8 L USM + EF 70-200 F4 L USM + EF 1.4x II + SlingShot 300AW + 055XPROB + SBH-100
If you know anything about photography you have your answer already! Any lens is just as good as the one that is operating it! Personally I like mine for teleshots (Nikon 300/F4) with 1.4 convertor and it works GREAT....providing the circumstances are OK to use it!!!