Forget the incredible but monstrous 600mm f4L IS, and opt for this fabulous optic instead if portability is an important factor. Its weight is 70% that of the 600mm, it has the same lens speed, and the difference in focal length is almost unnoticeable in the field (It's like comparing 10x binoculars to a 12x).
Anyone who tells you that the 600mm f4L IS can be hand-held is having you on. For me personally, although the 3lb drop in weight is definitely noticeable, it's the difference in bulk that is the most significant factor. The 600mm is just plain enormous, and an absolute brute on the back!
Although it is border-line hand-holdable, it is still no featherweight, so I would definitely recommend opting for a decent monopod to make use of its full potential. As other users have stated, it's razor-sharp to the corners even when used wide-open. A sheer joy to use!
Over here in Japan we have a glut of used equipment, but after 3 years, I gave up waiting for one of these lenses to appear in the second-hand shops. That speaks for itself.
When used in the field with one of Gitzo's carbon-fiber monopods and a 1.4x extender,(and as a recent addition, the Canon EOS-1D Mk111) it is a very efficient bird/animal setup, but a combination of Wimberley's WH-200 head on a decent set of Gitzo legs is really the only way to go. Nothing, and I repeat, NOTHING, goes close to the Wimberley for bird work. (As I'm sure many bird-watchers would agree)
I couldn't possibly recommend anything higher than this spectacular lens (that is, apart from the 300mm f2.8L IS USM !). Another first-rate Canon product, and worth every cent. Canon EF 300mm f/2.8L IS USM Telephoto Lens for Canon SLR Cameras

I bought this lens fronm B&H Photo Video. The service is excellent and the lens is awesome. The best lens for nature and sports photography. Un paralleled in image quality.

Sandeep.

I am very limited on my picture taking knowledge, but bought the Canon EOS Rebel XTI and want more lenses. When I read what people have taken time to write in about I have to say I feel better educated before making a purchase and want to THANK all of you!

I take pictures at weddings. So, inside shots are a must and evening light for receptions. I bought the Canon Speed;ote 430EX flash and LOVE it. Easy to handle and not blinding everyone as I expected. Easy to put on and take off.

Then I bought the Opteka Bgrxt Grip and LOVE IT! The camera is small and yet with the addition I feel safe holding it in any position. I read where others had problems with it not being tight but my fit is perfect. I also like very much the battery storage allowing me to shoot without changing anything.

My next purchase.....lenses. I want to take outdoor shots of wildlife and notice the EF 100-400 mm constantly mentioned. I am going to bite the bullet and purchase this item soon.

PLEASE anyone out there reading my note, please suggest another lenes for weddings. The past wedding I used my new gadits and my pictures were better than the person hired to shoot the wedding. I have an eye for taking pictures but am not a smart cookie when it comes to knowing what lenses to use. The numbers, settings confuse me totally.
THANKS!

I needed more reach than 400mm gave on my EOS 1D, so I purchased Canon's 500mm f/4L IS three years ago. It's incredibly sharp out to the corners at f/4. With bird and mammal portraits, you see every feather, every hair.

The lens is light enough to use on a Wimberley Sidekick. Some claim to be able to handhold it, but I think its eight-plus pound weight is a bit much for that. I carry mine in a long lens case with backpack harness from Kinesis Gear.

This lens' image stabilization works very well. I've used it on a tripod with one-second shutter speeds and good telephoto technique, and gotten sharp, publishable images.

With an EOS 1D or 1Ds, lens and camera are rain-proof in mild storms.

I also got mine through Adorama, and had no problem with delivery or post-sale advertising.

If you buy one expensive long lens for wildlife, choose this one.

I've bought several lenses from Adorama, two of which include a new 300/2.8 IS and the other, this 500/4 IS. All of their deliveries were prompt and well packed. I have zero complaints with them. I dealt the John (Green, I think is his last name) via email. I highly recommend them.

As for this lens, it is incredible. Wide open it creates a wickedly sharp subject against a wonderfully soft background (good bokeh). AF is very fast. This lens is extremely good and will still AF when used with the 1.4x converter. (AF works up to f5.6.)

I normally don't take the time to review products online, especially if there's plenty to read already. However sometimes I buy a product that is so exceptional, I feel it deserves a review. This is definitely one of those products.

This lens complements my series 1 EF 400mm F2,8 (non-IS @ 15.25 lbs). There was nothing wrong with my 400mm, but it is a monster to use. As I started to get more active in motorsports and wildlife versus low-light indoor sports, the 500mm F4.5 was very tempting. But I never bought it, opting to rent instead.

I really liked the 500mm F4.5, but always wished it could be an F4 as well as have IS. When Canon released the updated 500mm F4 IS, I knew I wanted it, but the price was very steep. Eventually I saved enough to buy it and am happy to say it was worth the wait.

It's half the weight of my 400mm at about 7lbs, which makes hand-holding it for moving subjects quite a possibility. The amount of time you can do this is up to your stamina, but this really does open up new possibilities. As much as I like using gimbal heads with ultra-telephotos (the Bogen/Manfrotto gimbal head is another product I highly recommend with this if you think spending $600+ on the competition is highway robbery), there are tiems you really need the responsiveness of being hand-held. The 500mm F4 affords this capability.

The extra light gathering ability of the F4 version means your 1.4x extender yields a reasonable F5.6. 2X @ F8. I used this lens with the first-gen Canon 1.4 and 2x extenders and image sharpness is very good to excellent. I can only imagine the series II versions are even better, which I will eventually upgrade to.

The IS functionality is simply a godsend for this focal-length. I was amazed at how tack sharp some of my hand-held images were on static subjects down to 1/90th sec. This would be impossible with the non-IS F4.5.

The new F4 is even sharper than the F4.5 version.

The control layout is like other Canon ultra teles...logical and well laid out. The AF function buttons on the lens barrel are awesome. I like to use them to switch AF from AI-Servo to One-Shot.

Of course the AF preset is a excellent feature as well.

It's a pricey piece of glass, but at this level you are buying essentially a hand-made lens which is not mass produced. If you see this video of how they make this exact model, I think the price is absolutely justified:

http://web.canon.jp/camera-museum/tech/l_plant/index.html

The 500mm focal length is just right. Long enough for wildlife, but at half the weight of the 600 F4 but with almost the same reach. This version is the cream of the crop of the 500s IMHO.

What's amazing is that it can fit in a small photo backpack. I use a Lowepro mini trekker classic and this lens fits perfectly with a 1Ds, extenders and extension tubes, and even an 85mm F1.2!!

Kudos to Canon for really going all-out on this lens!!

This is a magnificant lens! I'm still learning this beast, but I certainly don't regret buying it. With the 2X teleconverter on a sturdy tripod, it gives me an effective focal lengh of 1300mm (500 X 2 X 1.3 - conversion factor for the Canon 1D Mk III) - you'd have even more focal length with a camera with a larger conversion factor - 1.6 for cameras like the 30D, but in most instances that would be too much FL. With out the 2X TC, the AF is lightning fast, and even with it it's pretty fast. You only get the use of the center sensor with the 2X, so if you're working small subjects like humming birds, it works well to focus on the eye, lock focus and recompose. With a little practice, this lens will give you amazing sharpness. If you're a serious bird photographer, this will be your favorite super telephoto. It's lighter and cheaper than the 600 f/4 and you're only giving up 100mm. I've only been using this lens for a week, and I absolutely love it!