Canon EF 800mm f/5.6L IS USM, reviewed by johnf3f

Please excuse the quality of the image!
What can I say? It's a bit of a monster and most would consider it too specialised to be very practical.

This lens, bought brand new, costs a small fortune as you would expect considering it's Canon's longest lens (currently).
I bought mine secondhand to replace/supersede a Canon 600 F4 L IS Mk1. My main considerations were longer native focal length and reduced weight.

Pros
Relatively light for it's focal length and aperture.
Very sharp and detailed images wide open
Good Bokeh is easily achieved, due to the small DOF - in fact it can be hard to avoid!
AF is quick and accurate, not in 300 F2.8 territory, but very good for an F5.6 lens.
Reach - the combination of focal length and excellent IQ make this the ideal lens for birders who like frame filling images that are full of detail.
4 Stop IS. The IS system on this lens is very good having been developed not long before Canon introduced their 3 mode IS system. Having said that I don't use it anyway! Whilst IS can be handy AF and tracking work better without it, so I keep IS in reserve.

Cons
Price. Whilst I was lucky with mine these are seriously expensive lenses.
Weight, while it is light for what it is it is still a heavy lens (4.5 Kilo) so you will need to invest in quality support which is more expense and weight. Hand holding this lens is possible and quite amusing to watch!
Extenders. With a 2 x extender you loose AF and the IQ is not the best. The F4 Superteles are better in this respect. The 1.4 extender works OK but there is a loss of IQ and AF is slow - though accurate.
Aperture. This is an F5.6 lens (and F4 would weigh a ton!) so you will need a camera with good high ISO if you are going to shoot in less than ideal conditions.

Overall I am delighted with this lens and use it for 80 to 90% of my photography. Though it can, on occasion, be too long - this happens rarely in my experience.
Good accessories are essential with gear like this so things like backpacks, tripods, heads etc should be selected carefully. For reference I normally use a Lowepro Supertrekker AW2 pack (no longer made) and a Gitzo 3530 LS + Wimberley 2 for support.
If subjects like small to medium birds are your fare then this is the one to get. The Canon 600 F4 L IS Mk2 should also be considered.