Personally, I've considered the EF 28-300mm f/3.5-5.6L IS lens on several occasions, for convenience and not having to change lenses. I have the 100-400mm, so I'm comfortable with a push-pull zoom, and unlike the 100-400, the 28-300 is weather-sealed. But every time I've come close to pulling the trigger, the optical compromises stop me. Still, the convenience remains tempting (although a 2-lens solution of 24-105mm + 70-300mm L seems more likely for me).
The 28 - 300mm is wonderful providing you can stand reasonably still and use a monopod, and don't mind people staring at the big white lens (but you know that from the 100 - 400mm). It's particularly useful when you just don't know what focal length you'll need and there isn't the time to change lenses or swap cameras when using two bodies. One example was an old steam train, the only reasonable place was on a bridge, and with a superzoom you get the opportunity of many more shots as it comes towards you. Perfect for video where you just can't change midstream too.
Optical quality is surprisingly good, certainly as good as the 24 - 105mm and well ahead of commercial requirements. If you absolutely have to have the ultimate then it's not for you, but you'll miss so many shots acheiving it that it might not be worth it.