Ive been reading here for a while and there is a lot of good advice that I have read, thus I want more! I know most of you peeps shoot with mostly L glass, so my desires below might be a little low end but as a broke college kid i can only afford so much. I have and XSi, 18-55 kit lens, nifty fifty (been having a lot of fun with this in low light) and some tamron superzoom that has less contrast than a fog bank. I will not be a pro, just a someone who likes to shoot stuff and has another 30 years to get the perfect kit together.
Here is the deal, Ive been saving up for a while for a 70-200 f/4L IS, as I convinced myself that I needed it more than oxygen and was going to finally pull the trigger. Wicked lens I know and sharp like a fox is quick. Then I thought about how that lens would probably get used roughly 10-20% of the time. I also want the EF-S 15-85 (I want the expanded range over the 17-55 fast apature for a walk around lens and cant justify the price for a 17-55) and the Sigma 30 f/1.4 or Canon 35 f/2 (an appropriate length on a crop rather than the nifty, and still the low light/DOF that I am having fun playing around with).
I bought a 70-200 f/4L NON-IS at my local camera shop (I have since learned that this shop is less than decent) and returned it a week after because I wasn't happy with the sharpness. When I returned it, the guy processing the transaction laughed and said that THAT lens has been returned a number of times for the same reason so I am not sure that I want to completely write off the non-IS version.
I also know that I will get a new body within the next 2 years (I want higher than 800 useable ISO and better AF, dont care about higher MP), thinking whatever replaces the 7D or 60D and maybe whatever the low end full frame is out at the time. With the FF, I know that the 15-85 and Sigma will be incompatible and am not opposed to selling them later down the line.
Here is what I can't decide: get the 70-200 f/4 IS and only have mediocre everything else, or get 2 of the others (70-200 f/4 non-IS, 15-85 or fast-ish prime) and have a more complete/quality range knowing I will probably get the 70-200 IS down the line.
I'm leaning toward the 70-200 non-IS and the Sigma 30 f/1.4. I know the 15-85 would get more use than the 30 f/1.4 but would not let me learn DOF and low light as much. I'm also thinking that the 30 f/1.4 would change my mind about the 17-55 or maybe twist my arm into a 24-70L.
I'm just not sure what to get but know that I want more than the kit lens. Go forth and tell me how to spend my money!
With a APSC sensor I don't think you'll get as much use out of the 70-200 as you would the others. All of the 70-200s are great, but I think you would get much more use out of the 17-55 or 15-85. And although I've never used the 15-85, I LOVED the 17-55 (there is a reason its that much more %). IS works very well, f/2.8 all the way through, and razor sharp (I thought it was easily sharper than the 24-70L and definitely the best non-L lens that I've used).
I would seriously consider looking for a used 17-55, it's a great walk around lens ands very versatile. You could probably find one for $850-$900 used, I would recommend at least renting it for a bit before you buy the 15-85.
As for the Sigma, I used the Sigma 20mm f/1.8 back when I had a T2i and it was a nice focal length, but wasn't sharp at all and the colors looked strange. If you're looking for primes maybe try the 100mm Macro (non-L), as you can use it for portraits or close-up photography. Or maybe a used 85mm f/1.8 (300-350 used).
I'm not saying don't get a 70-200, I just think you would get much more use out of the 17-55 f/2.8 IS (and you could always get a 70-200 later).