Tamron, Tokina & Sigma have several very good lenses, but they also have some duds. I strongly recommend that you be very specific on what lens you're shopping for & how you intend to use it. (IE landscapes, action, low light etc). The more info you supply the better.
Like most lens manufacturers, it depends on which lens you are looking at. I have a friend that has one of their SP 70-300mm VC USD zooms that is pretty sharp. He hasn't had any problems with it and is very happy with the image quality. On the other hand, I have one of their AF28-300mm F/3.5-6.3 XR Di VC that I purchased as an alternative to the Canon 28-300mm f3.5-5.6L IS that I also own for when I want to travel light. The Tamron is much smaller and lighter but the image quality suffers quite a bit compared to the Canon L lens. Of course the Tamron is probably 1/4 the price and weight so it's a tradeoff. The VC (image stabilization) on the lens works very well but the images are soft if printed or viewed at a large magnification. The Tamron is amazingly slow to focus with a Canon 5D Mark III once the aperture gets to f6.3. Even at f5.6 the lens is no match for the more expensive Canon where focusing is concerned.
As a general rule the greater the range the lower the IQ (image quality) & the more it costs the better the IQ, but there are exceptions. Superzooms like the 18-200 & greater are a compromise, BUT many are very happy with them, myself included when it fits my needs (which can be cost or size / weight). I have the original version of their 18-270 VC & used it last year as my travel lens on a 7D; AND when I went on my annual vacation to Cozumel Mexico I found out there was going to be an air show a few days after my arrival. I used what I had & i'm happy with the results as would most people, but when you look carefully the IQ on the wide end is a bit soft. Very few people would notice or complain about thatsoftness and I won't either but I have better gear & a lot of experience shooting air shows, but they are radio control events using smaller & faster (considering the scaled down sizes).
The newer version of the 18-270 VC is supposed to be better, but I don't know whether it's the IQ or AF that's better or both, but this is the link to my results using the original version on a 7D (18 mpixels, fast AF system).
If you'd be happy with similar results & are looking for a 1 lens solution then a bit more research should confirm whether or not it's worth a try.
I was just reading a photo magazine at lunch today, and it was testing "affordable telephoto" lenses. The Tamron 70-300 came out on top in that "contest".
What lens were you looking at?
I can't vouch for any of the Tamrons but I can say some very good things about the Sigma 50-150mm f/2.8 EX DC OS HSM APO and it's cousin the Sigma 70-200mm f/2.8 EX DG APO OS HSM.
Right up front neither of these lens' are “L” level or quality. But they are pretty darn good.
I know of some problems with certain off-brand lens' so make sure the one you choose works with your camera. Sometimes you can send it back to the manufacturer and have it re-chipped to work but who wants to do that?
Stick with Canon if you are able to but in my case Canon does not make a 50-150mm lens for crop cameras.
The New Tamron 70-200 2.8 and 24-70 2.8 VC's are pretty good from what I've seen and heard. Throw in a Sigma 35mm 1.4 and you've got a sweet 3rd party kit.
09/26/2023: New firmware updates are available.
08/18/2023: Canon EOS R5 C training series is released.
07/31/2023: New firmware updates are available.
05/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
03/30/2023: New firmware updates are available.