As you can tell from the rest of this website, I'm a computer programmer by trade. I don't build roads, maintain power stations, clean rifles, or rescue people from automobile wrecks. So my practical requirements for a multi-tool are low. I might pan a MT that somebody else relies on every day for tasks far greater than I ever have to think about accomplishing. So just keep that in mind when the read the below: beyond the basics, what I care about most in a MT is the engineering that went into it, not necessarily how much you can beat at it until it dies. That's a perfectly valid (and some will convincingly argue, superior) criteria for what makes a good multi-tool... it's just not mine.
I'm also not much of a steel snob — this usually doesn't make much of a difference in multi-tools anyway — so for me the difference between a Skeletool and a Skeletool CX is negligible. Edge retention and field maintenance is important, but if you're using that as your main point of comparison between a Gerber Diesel and a Victorinox Climber, I think you're missing something.
In my mind, the undisputed king of the multi-tool is Victorinox. Their craftsmanship is superior, in every single tool. The tolerances inside a multi-tool are incredibly tight for what the user will demand of it, and Victorinox never fails to make precise use of every single millimeter available. What's more, the level of fit and finish in a Vic tool is second to none. And I speak not only of the well-known Swiss Army Knife (in its many varieties of size and tool load), but also of the venerable SwissTool and Spirit. Some may doubt that the SAK is a multi-tool, they certainly cannot deny the SwissTool its rightful place. In my opinion, this is the most brilliantly engineered multi-tool there is. Of course, I do consider the Swiss Army Knife quite a capable multi-tool, and one with a tradition of design and production excellence.
Second only to Victorinox is America's most famous MT maker, and perhaps what most people think of when they hear the term "multi-tool." Where I'm from, anyway, the terms are almost interchangeable for normal folks (that is, people who aren't MT connoisseurs). Actually, a Leatherman was my very first multi-tool. When I bought my house, I told my girlfriend that I'd always sort of wanted a multi-tool, and she encouraged me (as she has encouraged me since). So I did some research and found that the LM Wave was among the best loved. I bought one, loved it, and it's still in my bag every day.
Leatherman's quality control isn't perfect, but rarely have I seen anybody who's had any kind of problems with their tool right out of the box. The fit and finish isn't nearly as good as Victorinox's, but that just means you won't have any trouble getting that steel all nice and dirty.
I've got a few SOG multi-tools, though not nearly close to the number of Leathermans. My feeling is that the SOG tools I have are generally not quite as strong as their Leatherman competitors, but that the fit and finish is a little better. One thing that stands out among SOG multi-tools, though, is their gear-driven compound leverage. This unique feature increases the force used to close the pliers. It also allows you to force the pliers open, which is an interesting and sometimes very useful possibility.
I seem to hear that in recent times, SOG multi-tools have lost some of their shine. While the specimens I have are all fine, there have been numerous mentions of bad QC on the PowerDuo at Multitool.org.
In my experience, Gerber is a mixed bag. And that's all I have to say right now because I'm going to go play Dark Souls. I promise to update this later to be more fair... in particular, I really like my MP600.