Depends on what you plan to use it for:
is it studioworks inside? then it is fine.
Are you going to hunt butterflies outside, then it is too slow, and worst, it focuses by extruding itself, and that is both difficult to work with and also scares the critters away.
For outside macro on live animals I recommend the nikor 105 VR.
I do not have the sigma 90 macro, but I have the 50 and the 150, and I have seen the 90 in action.