I might be one of the few people who is going the other direction. I had mine since 2010 and even used it with e 2x TC for a while, but in the end because I'm not a professional photographer, not only was the ideal focal range compromised by my crop body (7D), it was difficult finding scenarios where I wanted to bring along the extra size and weight (I actually had to buy a whole new camera bag to accomodate this lens). And good luck bringing it out in public for street candids without getting the paparazzi comment, or parents hiding their children, etc. Even if positive, unless you are at a sports event where everyone has one, you will always get SOME kind of comment just by taking it out.
Yes the IQ, focus speed, IS, etc is everything that everyone says. But none of that matters if you leave it at home or in your bag. I'm personally looking forward to going to primes so I can not kill my wrist/forearm walking around or feel like I need a flash every time the light isn't perfect. Have already borrowed the 85 and am satisfied both with the performance as well as having an extra $2k in my pocket.
I wasn't into birding previously but I was forcing myself to get into it to justify the cost of the gear, which IMO isn't a healthy way to go about the hobby, so I guess my 2 cents especially to the hobbyist is that it depends on whether you think you will actually use this lens and get your money's worth.