I wish I did have one however, like you, it's probably going to be my next purchase lens-wise.
Although, I have used one coupled with a 2x extender (so, 140-400 x 1.6 = 224-640!
)
I have the 70-200 IS f/2.8, which is normally coupled with the 1.4x (98-280 x 1.6 = 157mm - 448mm equivalent ... at f/4)
and the build quality and clarity are just unreal but then again, what can you expect from a Canon L lens.
Someone (luminous landscape?) has a "400 vs 400" report, where they conduct a shoot-off between the 70-200 with 2x to the Canon 100-400 IS f/4.5-5.6 L lens. My recollections are that the latter had a slight edge. It also would be a bit cheaper too.
The reason why I personally passed on the 100-400 was because I thought that it would be important/useful to have the capability to drop down to f/2.8 for some subjects. I haven't used that capability much yet, but its still there for when I start to need it
There's only two problems:
1) either lens is above the OP's price range.
2) both are telephotos, no normal or wide angle capability.
The real question here might be more along the lines of ... what are your near-term priorities, to which you may have the opportunity to add to the lens budget later on?
-hh