I have just bought the Canon 70-200mm F4 L.
Price was about £420 from Warehouse Ex but there is a £70 rebate if bought before 31/12. Making this a bargain £350 (Not sure of Ian, Kelso's price though)
Anyway, my intention is to find a second user mint Canon 1.4x extender at a later date. I have seen them go for less than £100 on the US market, so will wait until next year. (lol funds really)
This then will give me the budget end best of both worlds an excellent 70-200mm L glass for multiple uses and the addition of the 1.4x extender will give me the reach I want (roughly 100-280mm) for say Airline Photography or Wildlife (just at 1 stop expense)
I think this is the right way to go about it, on my budget. I used to have a 100-400mm L and to be honest I didn't like it. Didn't like the push pull tele action and I thought it a little soft.