Isn't "3rd Party" the one about to be led by either Kenneth Clarke or Iain Duncan Smith?
(UK political joke - to be ignored by our foreign cousins)
3rd party lenses have always had a bit of a poor reputation compared to Nikon/Canon's own lenses. This is not always fully justified, but it's kinda like saying "yes, well I know the new Skoda is built by Volkswagen, and is supposed to be really good, and costs 2 grand less than the alternatives, but I wouldn't want one on my drive".
For those considering *any* lens, you might like to check out www.photodo.com
, where they have MTF test results for a whole bunch of lenses from Canon, Nikon, Minolta, Pentax, Sigma, Tamron, Tokina etc. These won't tell you the whole
story (they don't tell you about flare, colour rendition, build quality etc.), but they will give you a good idea of the relative sharpness of different lenses.