cellomaster27 said:
First of all, no offense but I would sell the T2i and buy a newer, better Canon body before upgrading lenses. Certainly if you are considering buying a "super-tele" like the 120-300 f/2.8, or a Canon super-tele...you definitely simply must buy a better camera.
My experience with one copy of the Canon 10-22 was horrible. Very soft in the outer 60% of the image at all apertures. This was in 2010. I can't believe the lens is still in production.
I've not tried one, but from what I have read on here and at the rental websites, I would avoid the Sigma 50 f/1.4. Too many focus issues. Also, even Sigma's own published mtf chart shows a steep drop-off in resolution toward the corners of a crop camera image, and basically the whole outer half of a full frame image. If I were going to stay with the crop camera format, and I really wanted a fast 50mm lens, the Canon 50 f/1.8 ii is kind of a no brainer. It punches well above its price class.
If I was still tied to crop cameras, I would personally heavily consider buying the new Sigma 18-35 f/1.8. It could take the place of a lot of prime lenses, including Sigma's own new 30mm f/1.4. I tried the older version, and loved the color palette...but in the outer edges of the crop image area, it had some weird distortions, probably coma...at least when viewed at 100%. The bokeh was less than perfectly smooth at times, as well.
I have owned (including the 120-400 zoom I have now), three Sigma lenses, and rented a fourth (at least that I can recall right now). In my opinion, they all offered good-to-excellent value for money, which is more than I can say for many (but not all) Canon lenses.
I like the Canon brand a lot, but to spend more (usually double the price) on some of their lenses, when they don't offer better overall performance, let alone better value, is silly to me. Photography isn't about bling. That said, if the particular Canon lens offers something that you must have, that the Sigma lens does not...then that makes more sense. However, if that's the case, those people shouldn't sit back and spout that the Canon version is "better" overall, when in those situations, it likely is not.
Now that the Canon 100-400 replacement will likely be priced in the $2500 range (Canon simply can't stand idly by while Nikon sells their new 80-400 at that price level)...that will help hold the value of the previous version on the used market. But it just might also help support the value of competitors like Sigma's similar lens on the used market. I feel I made the right decision in all of my Sigma purchases and rentals, and have never regretted any of them. One of them even helped me get published in a national photography magazine in 2012. I can't say the same for some other brands. Even a recent Tokina purchase proved to be inferior to the sample I rented a while back.
Realize that much of Sigma's reputation for poor design and quality control of lenses in the past, was earned. But starting in about 2007, and especially by 2008-'09...they really seem to have turned things around. For example, lenses are no longer painted with cheap paint that flakes off. And the lens designs improved a lot.
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but considering budgets and my rather low budget, as much as I don't want to admit it, I am considering third party sources. Canon's prices are ridiculous. 
