Trust me, those thoughts all went through my head. IQ-wise, it's not too different from the 24-105mm - less distortion, actually, and similar sharpness across the range. A stop slower (it goes to f/5.6 so fast it might as well be a constant f/5.6). The main arguments against it are the size and weight, but as I said, I'm already used to those...
I do view it mostly as a walkaround lens for trips out with the kids. The visit to family was more of an excuse...if I were going somewhere I'd never been (family lives where I grew up, and we go fairly often), I wouldn't compromise on the lenses.
@Steve, 'need' is probably too strong of a word...
Like other lenses I was not certain were optimal for my needs, I bought this one used. I've mentioned my personal rule before - not paying more than 70% of current retail price. In this case, I bought it used from CL just 3 days before the last rebates ended (the 28-300mm wan not one of the extended rebates), and the price of a new lens went up by $270. So, that means I should easily be able to re-sell the lens for what I paid, if not more... I view it as a long term, essentially free rental - it's a great way to test out a lens.