I serious doubt that it is good as a Canon or Nikon. As good as a Sigma? ......perhaps
Lenses are a 10x better investment than Camera bodies
I recommend saving up until you can afford the Nikon or Canon version (depending on your Body) and you will end up keeping THAT Lens for years and years (and even many Bodies).
I can just about guarantee that if you REALLY are into photography, then you will end up selling off the Tamron (losing a bunch of money in the process, as they DO NOT hold their value) and buying the Name-Brand Lens anyway. Also the focus-motors are not as quiet as a Name-Brand lens and they are not made near as well (remember you get what you pay for...)
If you can't afford a new one, try the Fred Miranda.com site, there are used ones there all the time..
If God hadn't meant for them to be sheared,
he wouldn't have made them Sheep...