I'm actually in the process of ending up switching back a large number of systems. I have a few custom apps which involve an OS X front and back-end, and these are being ported / enhanced to run on Vista / future Windows and Linux respectively. I expect to have a gradual transition starting 2010 - with the XServes probably being decommissioned in 2011/12.
My re-switch (first time was at the start of the 90's) was caused by the release of the Intel Macs and the potential they represented. However the reality hasn't been so great in terms of the relative core stability of the server OS, some reservations regarding the desktop OS and also in a big way in terms of the engineering quality / quality control of the hardware. On the latter part I have to admit to some wishful thinking on my part since I initially believed to an extent in being able to field a hybrid OS X / Windows setup on Apple gear. As a development platform I can see many advantages of OS X. But at the same time my interest is more in a deployed solution.
There was also a social reason to switch development, and in the process to wipe the slate nearly clean it represented a good time to figure out what would actually work for us... and I decided that we need to cut a break from Apple, and just call it the failed experiment that it was. The social reason is our soon-to-be-ex OS X developers, who are entitled prima donnas to say the least. It is a drag that the switch will take 3-4 years, but there's first of all the money invested which I can't just throw away since we are actually working with the current solution despite the irritations involved, and there's also the manpower involved in such a major system re-jigging which I just don't have right now.
There's also the hardware that the OS runs on. As a designer I appreciate the attention to detail of form. But also an an engineer, to me the attention to detail in form in a consumer / professional product is nothing without the same attention to detail in matters of function as well as production. Apple do a very good job in many respects, but for me their lack of engineering talent is contrasted by their abundance of design talent. And if a manufacturer can't get both just so, for my working (and even to an extent playing) tools I'd rather have the engineers in the driving seat than the designers.
In terms of hardware and OS, I currently use the latest and greatest of both sides. I was a fairly late converter to Vista having only switched all of my Windows systems earlier this year, as I felt it wasn't ready for me to adopt in mid-07. But now, no problems and I prefer Vista to OS X, especially in the 64-bit iteration. There are many minor areas in which OS X is better than Vista in terms of presentation, but for where it actually matters - an everyday, all-purpose application platform - it's definitely superior to me. On like-for-like hardware it's more stable than Leopard (and also Tiger, I have to say), it is just as nippy as OS X if not superior in many respects, as a general-purpose OS it's more flexible, has a better choice of genuinely quality computer choices - not just pretty gear for the hard of thinking or those who try rather too hard to be with the 'in crowd' - as well as better peripheral support.
I don't see Boot camp as a panacea - anyone who has experience of a quality, stable built-for-Windows machine will realise that Apple + BC is not quite there, and that's even before we get to the engineering deficiencies / compromises-for-form of Apple hardware. On a Mac, I'm more of a fan of virtualisation - which while once again not as stable as a real Windows machine, represents a genuinely handy way to run the occasional Windows app on OS X - if causing anything but a Pro to transform into a Hibachi.
There are a number of apps I like using in OS X and I doubt it's fully going away, and I don't see why I need to ditch it completely. As things stand now I'll maintain separate computers for each, as I've done since I re-switched to Apple. I would however one day like to be able to virtualise OS X supported on Vista / future Windows, which would be a far more appropriate way to virtualise for me. For stuff that matters, there'll be Windows. For stuff that doesn't but I like using, there can be OS X.