What to expect:
Great image quality, fast autofocus, good performance indoors and outdoors, rugged build quality, smooth zoom and focus rings, all at a reasonable price for a lens in this class.
What not to expect:
Canon 70-200 2.8 IS MkII quality images.
You can't buy a lens like this and expect the same kind of pictures you would get from an L series lens, it's just not going to happen. However, you can expect excellent performance and pictures worth hanging in your home, or even pictures to be published in newspapers if you're like me.
I used this lens as a freelancing professional for two years, and it was fantastic. I could keep it mounted on one of my Canon bodies, sling it over my shoulder and not worry if it bumped up against something. Actually, the build quality is so good, you might worry about whatever this lens bumps up against. Be careful around cars.
The hypersonic autofocus motor makes life a whole lot easier when you're doing candid photography or sports. It finds what you're trying to take a picture of and focuses in lightning quick, all while being silent. With an aperture at 2.8, you can take pictures in almost any lighting situation. No image stabilizer, but that's nothing a monopod can't fix. The tripod mount comes right off because it's hinged, and I wish my Canon lens did that. Keeps you from having to dismount the lens and risk getting dust on the rear lens element or on your imaging censor.
It's a little heavy, but you have to expect that with good lenses. Quality optical glass has some weight to it. You're not likely to wear yourself out handling it, even if you're out shooting for a few hours.
Should you spend four times as much as this on a first-party lens with image stabilization? Not unless you have the bucks to burn and you're going to do this for a living for the rest of your life, and get a whole lot of money doing it. Even if you're a pro, this is a great lens and worth every penny. If you're thinking about shooting weddings or doing other professional work, and you don't have the money for Canon's L series glass, this is your alternative. Not to mention, it's a perfectly good alternative that will more than do the job, it will astound you how you got such great images on a 70-200 2.8 at this price.
Things to mind about one member of the competition: Tamron's equivalent doesn't have the hypersonic motor, which means it doesn't focus quite as fast, and it's also louder. Once you shoot with a HS, it's amazing how much of a difference it makes to you whether or not you can hear an autofocus servo activate.
Is it a good macro lens? Well, it's not bad, but it's not great. Don't expect National Geographic quality macro shots, but it does OK with close detail shots. I'm not sure there is a good telezoom in this range that is truly a good macro lens. Macro lenses are highly specialized, and they're also very expensive.
Bottom line:
Worth the money and performs exceptionally for the price. You won't be sorry with this purchase.