I struggled for weeks justifying the cost of this lens, heck it's expensive. However, I bit the bullet, bought one and all I can say is WOW! I use it on a 5D MKII and the sharpness is simply jaw-dropping, I've never seen anything like it before.
Is it heavy... Is it big... Is it worth it...
All of these questions are things I considered, but now I own the lens and have seen the results, I don't care about its size or weight. This is now the standard lens fitted to my camera. It's great for shooting birds, wildlife, portraits and just about anything except macro. Having said that, you can get within a distance of around 1.5 metres and when cropped the detail is just awesome.
Sharpness, IQ, colour, contrast and bokeh are all superb, I'm really pleased I invested in this lens and I'm sure I'll keep it for years and years and years.
Yes, it's a lot of money, but if you can bring yourself to part with the cash, I promise you, you'll be smiling from ear-to-ear when you see your pics, it's just a beautiful piece of kit.