It's a superb lens in many respects. Very sharp and well corrected throughout the zoom range, with very little chromatic aberrations (thanks to the fluorite element used in it). It's also quite close focusing... able to do about 1/3 life size on it's own. The Image Stabilization is excellent, too.... rated for 3 or 4 stops worth of assistance, it's also very fast. This is the least expensive Canon lens with "Mode 3" stabilization. That mode only activates IS during exposure... which gives you some idea how fast it is, if it can do its job in a fraction of a second. (The next cheapest lens with Mode 3 capable IS is the $6000 EF 300mm f/2.8L IS USM II... the 400/2.8 II, 500/4 II and 600/4 II all have it, too.)

The guys over at Lensrentals.com love to take things apart just to see what's inside. When they did a teardown of the EF 100-400mm II they commented that it was "the best built zoom" they'd ever seen. (But, too be fair, I don't think they've done a teardown on a Canon EF 200-400mm f/4L 1.4X IS USM.... and I suspect that $11,000 zoom is even better built).

I've got a few things minor complaints about the 100-400mm II...

... It's fairly heavy and large... a half pound heavier than the original version. And a 3.5 lb. lens on a 2 lb. DSLR becomes pretty noticeable after a few hours shooting! So sometimes a tripod or at least a monopod might be wanted. It's also fairly large "girth", might be a little tricky for folks with small hands to hold comfortably. I notice I get more cramping in my hands with it during long shooting sessions.

... Which brings me to another thing.... the OEM tripod mounting foot supplied with lens looks cool and is a neat idea... but ain't all that great. It's stylish and curvaceous shape doesn't work well with Arca-Swiss quick release plates. In particular, the plates anti-twist features don't mate up well with the foot. The entire tripod ring isn't removable (the way it is on many other Canon telephotos)... only the foot itself can be unscrewed. And Canon chose to put a thumbscrew on it that's difficult to tighten. The good news is that there are Arca-Swiss compatible, 3rd party replacement feet available. I know of at least three.... RRS, Kirk Photo and Hejnar Photo make them (I use the latter). These also attach more securely than the OEM, and are held in place with a hex screw (which requires a wrench to install... Plus I recommend some blue Locktite as a precaution).

... The 100-400mm II is not an internal zooming lens... it increases in length as you zoom it to the longer focal lengths. Because of this it also changes balance, which upsets equilibrium if using the lens on a gimbal mount. Still, it's not all that much and isn't really a big deal. Someone might not even realize it, unless they are accustomed to lenses that are internal zooming and focusing, so maintain equilibrium well on a gimbal.

... Because of the non-internal zooming design and the fact that much of the lens barrel is rotating rings... it really isn't very practical to use a LensCoat on the 100-400mm II. Personally I like to use those on large, higher priced lenses, in particular, as a form of protection (the camo doesn't hurt, either).

... Background blur effects aren't quite as smooth and creamy as some other Canon telephotos. In certain situations, it can be a little "coarse", much like the EF 300mm f/4L IS USM.

But there are a lot more things about the lens that I like, including it's overall performance and image quality, as mentioned above. In addition, I like...

...that the 100-400 "II" was redesigned with separate zoom and focus rings (the original 100-400mm had a single, push/pull zoom and focus ring). Now, some people have noted that the 100-400 II's zoom ring is given "prominence" over the focus ring.... It's bigger and the more forward of the two (sort of right where the lens most naturally rests in your hand). I like this arrangement personally... but it can be a little disconcerting if switching back and forth between the 100-400 II and many other Canon zooms that use exactly the opposite ring arrangement.

... The 100-400mm II also has adjustable tension on the zoom ring... ranging from completely free turning to fully locked in place to prevent the zoom from shifting at all. Nice to be able to lock it for storage or carrying... or set it with some drag, to personal preference... or none at all.

... As an L-series, the 100-400 II is said to be reasonably well sealed for weather resistance. It also includes a matched lens hood... of the newer type Canon has been providing, which is bayonet mount, but also has a locking mechanism. This particular one also has a neat little "door" in the bottom you can slide open to be able to adjust a Circular Polarizer without having to remove the hood. Unfortunately, all this makes for a hood that's even more expensive than other Canon OEM hoods (none of which are exactly cheap).... Don't lose or break it! The replacement costs $70!

It's worth noting that both Sigma and Tamron are also offering 100-400mm lenses now and that they sell for less than half the price of the Canon 100-400 II.

The $800 Sigma is a little bit smaller and about 1 lb. lighter than the Canon. But it has no provision for a tripod mounting ring at all.... so will be predominantly a hand held lens (good luck using a camera's tripod mounting screw, with a 3 lb. lens hanging off the front of the camera!). I haven't used it (and probably never will due to the lack of a tripod ring) but wouldn't expect it to have quite as good image quality as the Canon lens. Some feel that Sigma's HSM focus drive and OS stabilization are equal to Canon's USM and IS... but I can't say.

Tamron has just announced a 100-400mm of their own, though it's not yet in stores and info is a bit hard to come by. The only price I've seen suggested would make it the same or slightly less expensive than the Sigma (and about $1300 less expensive than the Canon)... but most places don't yet show a price and that may be incorrect. According to early info, it will be similar in size and weight to the Sigma (so, a bit smaller and lighter than the Canon).... and there is going to be an optional tripod mounting ring available for it, with built in Arca-Swiss compatibility. The Tamron 100-400mm will have their USD and VC (roughly equivalent to Canon USM and IS). Who knows about image quality... but I doubt it will match the Canon. I don't think the Sigma or Tamron is as well built or sealed as the Canon, either.