Considering an IS Lense

After a 3 year break, I'm interested in spotting again and I'm thinking about getting a new lense.

I currently have the Canon kit lense and the 70-300mm. I was thinking of upgrading to the 70-300mm IS.

I know I asked this before, but I can't find my old post. I was wondering if an IS lense is worth the investment, or does it make a difference at all? I used my unstabilized lense earlier this week to get Marine One, and although shakiness ruined some of my shots at 300mm, most of the shakiness in most of the photos was easily corrected. I think the fast shutter speeds I used helped conseal it also. I can be shakey sometimes, and I fear in the future my shots will be ruined if I don't get an IS lense.

Any thoughts?

I think that if you're going to go IS, at least get a lens with a different focal length than you have. I think that it's a waste of money to get the same one with IS.

Check out the 100-400 maybe?

The 100-400 is my primary lens for spotting. It is worth the money if you have it...though if you do a lot of non-aviation the 70-200 f/2.8 (new or older) is an amazing lens that I constantly wish I have in my bag. I'm actually toying with the idea of getting rid of my 100-400 and pick up the new 70-200. The 24-105 is also a very solid IS lens.
For what we do in spotting the IS is very worthwhile.

i shoot the 300 f/4 IS which accepts a 1.4TC VERY well so you end up with a 420mm f/5.6 IS (autofocus is slightly less than blazing fast but still very accurate). I recommend this setup if you know you'll be shooting long-range or prefer a tighter crop on short range shots. Almost everyone I have spoken to claims the 100-400 is very soft at 400mm, but no one complains about it at all...it's a great lens especially considering the zoom range and versatility. I also shoot the 70-200 f/2.8 IS....it really is worth every penny.

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Almost everyone I have spoken to claims the 100-400 is very soft at 400mm, but no one complains about it at all...it's a great lens especially considering the zoom range and versatility. I also shoot the 70-200 f/2.8 IS....it really is worth every penny.

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Having used the 100-400 for a few years on two bodies now (40D & 7D) I can say that 400mm on the 40D did not appear soft, but with the higher resolution on the 7D I definitely see it now. On downsizing the issue can usually be resolved away, but not always. I am dying to grab a 70-200 f/2.8 IS. I've used it a lot lately from friends or rental and I love it with a passion, even considering trading in my 100-400 for one. But then I'd miss those extra 200mm. :?