Tamron or Sigma 70-300mm lens?

Are these lenses any good? I see them going for like $100 all over the place, and I noticed they have manual aperture, and is a pretty compact for the size. Only downside I can see off the specs it's the f5.6 aperture at long end(seems a bit slow for wild life, really wish it was f4.5 at 300mm).  That said I notice these seem to be popular among the Nikon and Canon crowd, are they good or just meh?