i want to stress again, if u do not need the versatility, it may be wise to carry 24-105 n 100-400 or 24-105, 70-200 etc. But if the situation demands only one lens and if u can use this together with the full frame 5D it is a miracle lens. Why 5D? Because 28mm on a crop body is not wide enough but again it depends on your situation. 28mm on a 5D is like 17mm on a 1.6body.
I shoot for events and there are times when i demand L quality and the situation is either too crowded or very very demanding and u need to get up and go in an instant. This is the lens i will bring along. U can't expect to change lens in an instant unless u r carrying two bodies. Theres an advantage in bringing two bodies but like i said, in a very very tight situation where u do not have much room or need to squeeze through, or do not want the hassle of two bodies, this is the lens again. Canon has designed this lens with careful consideration for those situation which i just highlighted.
If you are still contemplating whether to get it or not, look at your needs and situation. For me this is a luxury lens as it does not come cheap. It will come into very helpful use when the situation arise. The 28-300L is a very sharp lens at all focal length and i've tested and compared it to my 24-105, 24-70, 70-200, 100-400. It is not exactly a 70-200 but it comes very close. No other zoom lens of this range can match it in my opinion.