I have one and mainly use for motorsport. It's a very capable lens and as mentioned above the close-focus ability can be really useful - especially if you shoot things like butterflys etc. It's quite compact and I wouldn't say it was heavy actually - but it depends what you are used to.
Depending on your camera 400mm may be a bit short at times? If the reach is fine then I'd certainly go for it - great flexibility. I suppose another option to consider is the Sigma 150-600 C or S. I went with the Canon as I just felt AF was a touch faster and more consistent, also when it comes to resale the Canon's have historically held value better (not that I plan on selling).
Enjoy.
cheers
Az