Once upon a time, blog commenting was actually an effective link-building strategy (here’s Loren Baker recommending it back in 2007). But that really was “once upon a time” (at least, in internet terms…).
Today, excessive blog commenting executed purely to gain links is officially frowned upon. According to Matt Cutts:
What’s more, even if manipulative blog commenting doesn’t land you a very lovely penalty, thanks to the “nofollow” attribute that’s commonly used today, any value you gain in terms of link equity is pretty much negligible anyway.
But, that doesn’t mean that blog commenting can’t be used as alegitimate mechanism for marketing your company and generating leads.