I'm sure Zack and the pro's will be along, but I really think it depends on the particular lens that you are interested in.

Both of those manufacturers make some very highly regarded lenses, and some, not so much.

Do you have a particular lens that interests you?

I couldn't have said it better.

As a general rule, they're considered a low cost alternative with all that description implies. Every now and then they crank out a winner. The Tokina 11-16 comes to mind, as well as the 150-600 by the various makers (depending on your point of view, and what you need a lens to do - FredMan does great stuff with one)

Edit - the sigma art lenses seem to be well regarded among people that use them as well

I had the sigma, it was fine. Image quality, nothing wrong with it.

It's not weather sealed, and doesn't even have the little o-ring on the back to keep the dust down.

I found a smoking deal on a Nikon 70-200 and the sigma went byebye.

Eventually the Nikon went byebye as well for reasons

The 70-200 was sold off to pay for the 80-400 that I've been using lately.

Without hijacking too far off the subject at hand, if you are going to lose 2 stops in that lens swap, why not a 2x teleconverter?
You lose the same two stops, but have a better base lens in the 70-200 ??

You'd think that, but there's a lot of sharpness lost through the TC.

I tried that first, but even with AF fine tuning, it was never as sharp or as clean as I'd like it to be. Since the 70-200 wasn't long enough, and I wasn't really ever using the wide open aperture anyway, I decided to gamble on something better for what I was doing.

I'm gonna need you to provide at least one credible source for this claim.

What you think you saw or what you read on a forum somewhere isn't a credible source. So..here's the actual truth:

Nikon has made their own lenses the vast majority of the time. There have been a few known and verifiable examples of them contracting stuff out (a few lenses back in the film days come to mind), but nothing credible that I can find relating to Tamron, and further, Tamron is only mentioned in this silly rumor and there has been no evidence to substantiate it.

The Tamron lens you're talking about does have a lot of similarities to the Nikon version, but the coatings are different, and the image quality is *very* different. The "Tamron makes Nikon's glass" rumor was mainly passed around by shifty salesmen trying to fool people. Sometimes (apparently) it worked. A more plausible explanation is that Tamron could have done what many companies do, copy someone else's stuff but change just enough to avoid getting lawyers involved. That's how the third party lensmakers survive. They have to reverse engineer the mounts and electronics just to make the lenses talk to the camera. Is it such a stretch that they wouldn't use a similar design internally?

I'm glad to see you posting and participating here in the forum, but we try to keep the information here as factual and relevant as we can. There are a lot of newbies that read here and come to learn stuff, crazy stories like this don't do much to help them out.