The good news is that this lens is capable of producing some very sharp, quality images. The bad news is that it depends on your copy (thus 4 stars instead of 5). Don't necessarily believe those who tell you that you have "bad technique".
I went through 3 lenses before finally getting one that performed as one would expect for a $7,000+ lens. The first copy had obvious defects, produced soft images lacking detail, and was returned to the seller. The 2nd copy had something resembling bad chromatic aberration (also with soft images lacking detail once again), and went in to the California Factory Service Center twice. Both times, the FSC returned it to me in worst shape than when it went in. After contacting a Canon executive, my lens went to Canon's engineers and I was issued a loaner within 24 hours (something the head of the California FSC never offered). Canon's executives obviously care; I'm not so sure about their FSC.
Canon eventually told me they were going to use the lens to find ways to improve their manufacturing process. They offered to let me keep the loaner they had sent me, but I declined due to visible metal casting flakes on the outermost element. Canon ended up sending me a copy that was first inspected by their engineers. I've had that copy for 5 months now and it has performed superbly - sharp, good contrast, etc.
So if/when you buy this lens, buy from a seller you trust, and a seller that you know will work with you. I bought from Canoga Camera in southern California. With the first lens, they were supportive. With the second lens, they told me I could not return it, and then had their Canon sales rep call me. The sales rep insulted me (literally), told me I did not know what I was talking about, stated, "Are we done here?", and then essentially hung up on me. I was on my own when I contacted Canon executives in NY. Fortunately, they were extremely professional and responsive, but it was a long ordeal that was quite stressful considering how much money was at stake.
Using live view at 10x, the visible shake of 840mm (600mm + 1.4x TC) is evident, even when the lens appears to be steady. Yet with IS, the results can be impressively sharp.
In addition to a very sturdy tripod and good tripod head, I have gotten excellent results using the molar bean bag filled with 20+ pounds of pinto beans. This lens produces much better images than my Canon 100-400mm f/4.5-5.6 L IS lens. Even with a 2x TC, the results from this lens are superior to my 100-400mm.
For reference, I own Canon's 24-70mm f/2.8 L, 70-200 mm f/2.8 L IS, and 100-400 mm f/4.5-5.6 L IS. The 600mm is my only prime, so it should come as no surprise that it performs the best of all the lenses in my collection. The 70-200 is a close second in terms of performance.