At a nice dinner in Geneva yesterday, a local customer pointed out that the most visited websites in the world do not use any of the "usual suspects" among the CMS vendors. Mediawiki is the open source engine behind Wikipedia, while Blogger and WordPress are popular blogging platforms. Among the top 30 sites today, the remaining 27 use systems that are propritary and home-grown.
Interestingly most CMS vendors consider themselves worthy of the classification "leading", and many other analysts frequently crown winners; "strong positives", place them in quadrants or use other horserace style approaches.
Some might argue that those 3 systems placed among the Top 30 are not actually web content management systems. That may be so, but they do enable effective web publishing. Perhaps you don't need a CMS to power your website after all?
I would argue that you should look at your own requirements and not too much at the labeling used by the vendor or open source project. If you are a high traffic site, I certainly recommend extensive testing and many reference visits before procuring a commercial CMS.
You can find detailed evaluations of all 3 systems in the Enterprise Social Software Report.

I’m quite sure that customers no longer look for “leading” vendors, as they do not look for the “best” product. Fortunately, customers are wiser than that. I rather suspect vendors using these superlatives to risk losing business because customers see them as untrustworthy ;-)
Andreas, I do hope you are right, but I have my doubts.
In these times with financial turmoil, I’ve talked to many customers who are looking for “safe” choices, e.g. IBM, Microsoft or Oracle. As you already know doing a CMS project with any of these can indeed go quite well, but depending on what you want to do, it can also be quite a stretch to make things work.
History unfortunately also shows that the “safe” vendors, have so far not been so friendly on upgrades, e.g. MS CMS 2002 -> MOSS 2007.
You also have a good point about not looking for the “best” product. Rather I would encourage buyers to look at something that is “good enough”