At the time I thought it was a lot to give up for a closer - still kind of do, but the way Kimbrel has performed this year and the way the market for good relief pitching has boiled over since then (look at what Theo gave up for a 3-month rental of Chapman, among other examples) makes the deal a lot easier to swallow. Ultimately I think it will probably belong in the same bucket as the Sale trade, where both teams will likely end up pretty happy with what they got.
The real f***-up, in retrospect, was not signing Miller before the 2015 season. That was a perfect opportunity to solidify the back end of the pen for a few years to come, but Cherington seemed to be relying on Koji to continue being immortal at that point without any real plan to replace him.
Also, I have the biggest pecker in this discussion.