does anybody know if the 120-400mm lens for a Canon 550D is any good for kiteboarding photography?
cheers
Canon EF 100-400mm f/4.5-5.6 L IS USM would be my recommendation but at ~$2300 it's not cheap.
At 400mm your really going to appreciate the IS and a monopod to up your usable shots.
I assume you meant the Sigma 120-400 (~$10000... in which case i'd be looking at the Bigma (50-500) instead if your after something a bit cheaper than the Canon 100-400 IS. The 120-400 isn't bad, but the Bigma is used a lot more widely especially for surf shooting on a medium budget.
You might be better off using a 70-200 IS and a 2x TC when you really do need the reach.
edit: Heres a decent comparison between the lens that i've mentioned in this post:http://www.juzaphoto.com/eng/articles/canon_100-400_sigma_120-400_150-500_50-500.htm
Never used the Sigma 120-400, but have the Canon 100-400 as part of my kit, & for it's price, it's a nice versatile lens. Have shot at 400 hand held with IS on, & the results aren't bad if you have a steady hand & shooting at a fast shutter speed. But you are better off investing in at least a monopod as koma said, especially when shooting in windy conditions...
markg.com.au
if you want the best-of-best compromise..
buy a used canon EF 70-200L f2.8 lens, don't worry about the IS crap, get a 1.4x or 2x teleconverter and you've got a great f4 400m lens and one of the best lenses available
Get used to using a monopod or shoot as higher speed as you can and IS does nothing for you, infact you need to turn it off anyway when using a monopod or tripod anyway and if you learn to hold a camera right you won't need it.
and shooting portrait stuff at ~100mm range is spot on..
if your shooting flat water, and can get close to your subject, then the 120 to 400 is Ok ,
but for decent surf shots shooting from the beach you will need at least 500 to 600, (unless its a shore break), preferable the longer the better for surf shots,
dont bother with x converters as you will blow out your Fstops and quality goes to hell.
Another way is to use a crop sensor camera eg If you use the canon 7D it has a 1.6 crop, so with a 200 mm zoom L lens so you will get in effect a 320mm lens ratio
if you want to get in the water with your SLR I have a dive bag that will take a bigish lens. if your keen I'll sell it to ya for $50. (worth $400 new!!)
thanx heaps guys, very much appreciate your good advice and feedback.
im still not sure which lense to go for 
i think i will wait for a good 2nd hand option..
do you have some photos you have shot with the different lenses you can post up here?
MB
This is with the 70-200 f/4 L which is the cheaper L series and goes for about $1300 or so. You can only put a 1.4 extender on this or you'd lose the auto focus but that would still give you 280mm and if you are using a 550D which has a 1.6x crop on the sensor, its the equivalent of a 440mm full frame.
