Just a caution or two... ; First off it's already been mentioned that it's not actually lighter. ; It's actually slightly heavier than the non DO 70-300 IS, the perceived lightness comes from it being wider but shorter. ; It's more about being compact then light.
Also beware that it has a reputation for unpleasant bokeh, or out of focus areas. ; This has been a controversial issue, with some users upset over it and others not seeing it at all. ; The following is a link to an image that demonstrates the effect. ; I gather that this is a result of the DO optic element.
http://aesop.rutgers.edu/~rabin/Personal/Canon_70_300_DO_IS/slides/11_FresnelOOFAreas.htm
I haven't used it, but from what I've seen from googling it it sounds like it's kind of a quirky lens. ; For me the price factor completely eliminates it, but for those that it doesn't I think it's a case of what it provides and what it demands in return. ; It offers the more compact size that lets some feel less conspicuous, plus faster AF with full time manual operation as an option. ; In return it demands more post processing work, the accounts I've found say that images shot with it can need contrast adjustment, sharpening, and custom white balance work. ; I've actually seen a specific sharpening plug in plugged as working well with this lens, I've never seen a specific plugin referenced for a specific lens before.
One interesting note, two reviews I've read of it say that it works best with no UV filter on it, they say to shoot it "naked" at all times. ; This is supposed to relate to it being more flare prone, in flare situations the extra glass of the filter can interact with this unwanted light and create a loss of contrast in the image by brightening the shadows. ; I know that's scary to some, but the demo of the effect that I've seen is fairly convincing:
http://www.fovegraphy.com/70_300DO_TipsE.php
It's the first image, mouse over to see what you get with a naked lens. ; There's some further hints on post processing there as well.