I own a 100-400 IS which I think is great value. After reading Michael's review of the 400/5.6 prime and his comparison of that lens with the 100-400 zoom, I decided to order this prime. It's not the sort of lens that would be carried in stock, so it wasn't possible to order it for the purpose of testing it before buying.
However, when I did test it, after buying it and taking it home, I found it to be worse than my copy of the 100-400 IS zoom.
I went to a lot of trouble carrying out rigorous tests; printed them out as well as recording the tests on a CD-ROM and handing them to the shop. There was no question they would refund my money, but before they did, the supervising salesman suggested they replace the lens; get me another copy. I agreed on the basis that if any improvement was not greater than marginal, I would still want my money back.
What surprised me was, instead of replacing the lens, they sent it in for calibration and adjustment and gave me back the same lens.
There was a slight improvement, but it still wasn't as good as my 100-400 at 400mm, so I took my refund.
On reflection, it was an annoying experience because it showed a certain lack of enterprise and a degree of slackness on the part of the retailer. It would have been really encouraging if someone has said, to the effect, "This is not good enough. This lens should be better. If you still want one, we'll get you a good one for sure."
This was my first lesson in lens buying. Always try first. If that's not possible, test later and insist on a refund if you're not satisfied. Of course there's a danger you could become an unwelcome customer if you return too many lenses. It's a real problem.