I love the hell out of mine. It's
the bird-in-flight lens, but it's not exactly a general-purpose lens. A better choice for a variety of subjects would be the EF 100-400L.IS USM, since it has zoom and IS.
Remember, without IS (or a monopod), you'll be limited to 1/focal_length as a shutter speed to eliminate camera shake blur if you're handholding. Typically with this lens, that means shooting at 1/500s or so. And you're limited to f/5.6 as your max. aperture. So, daytime and outdoors only is pretty much the rule with the 400L. And usually at iso 400-1600.
The other telephoto prime in this price range is the EF 300 f/4L IS USM, which is also slightly more general purpose than the 400mm. If you only think you need 400mm occasionally, then the 300 + a 1.4x teleconverter might be a better fit than going straight for the 400. If, otoh, you plan on shooting birds, 400mm is a better length, since it's harder to fill the frame with a bird.