Have flown with easyjet with kayaks and have to say they are brilliant. done a total of 9 return flights with kayaks and each time it has been great. So long as you prebook about 15 pounds each way then no worries. Only ever booked kayaks on paddle bag goes free and you get additional weight allowance so it covers kit and camping gear.
Air Canada: Were very good but would not let me prebook the kayaks on despite trying to, caused a little comotion at checkin both directions, price was reasonable but not a smooth operation.
American Airlines: Don't even bother, flown with them a 12 times on buisiness and that was bad enough, stewards are dragons of the air in my book so I would not dream of attempting a kayak, but have no experiance of this.
RyanAir, have heard plenty of people who have travelled with them with no problems but no direct experiance.
Rating of flying with kayak, includes all aspects check in staff, deptarture time etc.
Easyjet 10/10
Air Canada 7/10
Rating of airlines without kayak but with climbing gear or other sports gear.
Mexico Air (aeromexico) 10/10
BA 6/10
emirates 9/10
virgin 8/10
American 4/10
Air jamaica 10/10 (more of an experiance)
RyanAir 6/10 (mainly due to delays but everything else was good.)
SouthAfrican Airways 9/10