I bought this lens month ago and I love it. I initially didn't want this lens. I was looking for Tamron 70-200 f2.8 VC and Tamron 70-300 VC to supplement my Tamron 24-70 VC but when I found this lens among used items on web site of my local retailer, I gave it a try. It cost me 70% of the new one and it still had one year of warranty. I also bought a tripod collar - Canon really can make money from nothing. The collar cost more than 50 f1.8! I somehow found the collar quite good for handling the camera with the lens.
My copy has IMHO little bit loose focus ring and IS makes a strange noise when starts and stops operating (it is only audible in silent environments) but otherwise it is flawless. I use it for animals (dogs, birds, zoo), flowers and close details in general. It has a very compact size (and pleasing weight) and fits to quite small backpack together with body, other lens and external flash.
I'm beginner and I still shoot with Rebel (and slowly collecting money for 6D) so I cannot compare IQ or build quality with other L lenses. This is my first one but it really make a great impression and I doubt it will be the last L lens I will ever buy (I already itch for 100L f2.8 ).
Edit: It is also lens which makes me think again about moving to FF. Extra reach on APS-C is a great bonus.
High quality glass is addicting. I strongly recommend the 100L Macro. It is also a superb lens. The loss of reach is certainly a challenge when moving to full frame. At this stage, at least, I find that the improved image quality more than makes up for it. My FF images are just much smoother in color transition, noise, and dynamic range.
Might I recommend that if you take the plunge to FF, hold onto that Rebel and use it when you want the extra reach. One area that can make a big difference above your Rebel is the ability to fine tune the AF to your camera body through AFMA. It made a very big difference on the 70-300L. It was good before; great after.