Sigma has been stepping up it's game...
Their 24-70mm is quite an awesome lens...
very noisy...but really sharp and great resolving power
I personally only own the Sigma 20mm 1.8... and I LOVE IT...(using it on a crop sensor)
Does vignette quite a bit on ff, but it's also a very very sharp lens.
Really? Since I got the 85 and saw how good it was I was considering this but the reviews are realt great
i wonder if a update is in the pipeline, if it came out in the same build quality as the 85 i would snap it up
Have you ever seen / read any of lightrules' (previously known as fstopjojo) lens tests? They are mostly years old, but he was pretty thorough comparing lenses, and he showed good results from the Sigma 20mm 1.8. He called it "El Hombre".
Here's the page of lens tests:http://www.pbase.com/lightrules/lenstests
The Sigma 20mm is tested in Popeye vs El Hombre and the 20mm shoot out. Maybe another one, too?
And to the original question, I'll agree that third party lenses are more of a compromise, rather than some of them being particularly "better" than a Canon or Nikon equivalent. Price vs performance or something that Canon/Nikon doesn't offer. If you buy one, and it turns out it doesn't work for you, the nice thing about lenses is you can buy them and sell them without much of a loss, if you're not afraid to buy used. Or you can rent one, if you want to buy new, and are looking at a big ticket lens.. For renting, just make sure you'll be getting enough hands-on use with it in the rental time-period.
A good compromise lens, if you ask me, as others have also mentioned, is the Sigma 85mm 1.4.. it's a pretty excellent lens (I have one). It's cheaper than the Canon by a LOT, but it doesn't have weather sealing. I don't need weather sealing, so that's not a problem for me.. Also, the bokeh comparison at the-digital-picture between it and the Canon 1.2 show that the 1.2 definitely has a leg up on the Sigma. But sharpness, they are pretty similar. I've never used a Canon 1.2, either, so I have no idea for sure.. But I reeally like my Sigma 85mm.
I also have the Sigma 120-300 2.8 (non OS). I bought it used and cheap, and I love it. It has a major weakness, though... chromatic aberration. I probably would be more annoyed about that if I had paid full price, but that's just me.. I otherwise love the zoom range and f/2.8 separation at 300mm.
I also have the Tokina 50-135 2.8, which I also got used and cheap (I can never say no to checking out a different lens, especially if I'll make money if I don't like it.. : D). It's autofocus is loud, but otherwise pretty good. And it's nice and sharp wide open, but I noticed some focus shift when stopping down around f4.. By f/5.6 it's fine, if I recall. Pretty much I just use it wide open. I love it's size, and it's available for pretty cheap used. I didn't know what to expect when I got, and I was pleasantly surprised by it. : D The crop-version of the 70-200's are a nice alternative to the full size thing.
I also have the Sigma 8mm f/3.5 fish eye.. When I bought it, it was the only one available. Now there's the manual focus Samyang, which is SIGNIFICANTLY cheaper.. And you really don't need autofocus at that focal length. : D I know nothing else about the Samyang other than I can probably never sell my Sigma without taking a huge hit, because of it, hehe. : D
There was also mention of Zeiss lenses and the Samyang 35mm. The Samyang 35 is on my list of lenses I'd like to randomly acquire at some point.. The main compromise with those guys is that they are manual focus.. The Samyang is nice and cheap.. don't need autofocus? Want to have a go with a beefy aperture prime and get comfy with manual focus at the same time? Then thumbs up to that Samyang! Just remember you need a proper focusing screen, and don't expect to take sharp pictures of moving subjects til you master manual focus. (They have an 85mm, too.. They also have a 14mm, but it has the wavey distortion.. which is hard to remove from hard lines.. architecture, and umm, maybe that's it? hehe..) The Zeiss lenses, on the other hand, are NOT cheap. So that's a serious decision to get a Zeiss lens. I've never used one.. but they have a serious following.. And they LOOK dang sexy, I won't lie.. But I have no idea how they perform. Even if they destroy a Canon L lens, you're still stuck with manual focus. Which is perfectly fine for some situations! Just the right situation.. Maybe people like Zeiss for video, too? Now I'm just making stuff up.. : D
The lens descriptions at lensrentals.com I find to be particularly excellent for comparing 3rd party to Canon / Nikon. They give a no-nonsense write-up of what to expect. They also specifically don't offer some 3rd party lenses.. I'm not sure if EVERY case is because of poor performance, but some of them are.. For example, there are stories in the descriptions how they dropped previous versions of some lenses because of unacceptable levels of malfunctions, etc etc etc.
Everything's a compromise! But I sure can say that my Sigma 85mm 1.4 is worth every penny I spent on it (which I didn't get used and cheap..). Will it stop working some day? Beats me, but I haven't spent 1 second even worrying about it.. (But I've had my 8mm fisheye for 5 years, and it's been 100%, so that's where I'm coming from.. I believe that lens had just come out, so it was kind of a "new" Sigma, rather than their older more-horry-story-lenses.)
Interestingly, I see a LOT of Sigma 50mm macro lenses on eBay that are listed as "autofocus does not work".. I have no idea if those are older lenses or what, but I see more of those than not. Maybe everyone with a working 50mm just loves it and holds onto it for dear life! A Sigma macro lens, for the right price, is also on my list of lenses to acquire.. If I were going to buy one new, I'd probably get the Canon 100mm IS L, but those Sigmas can go for $200-400, which is pretty hard to pass up for the performance.. I used to know the working distances of all the macros, but I don't remember.. I think when I fell in love with the Canon 100mm macro IS, I just stopped caring about the others, haha.. : D But still I wouldn't pass one up for the right price.
And really this needs to be put in perspective.. None of the lenses I listed are general-purpose lenses (OK, the Tokina is fairly general purpose, but I would say it's not nearly as versatile as the canon 50-250). But they all have their place. And the journey of picking that best lens for your situation sure can be enjoyable, but don't lose sleep over it, hehe. Been there, done that, haha. Seriously, whatever you have, you'll use and love. And if you don't love it, you'll probably get rid of it, and get something different! : D Everything's a compromise, really, haha.