I'm running what it is called a late 2010 Mac Pro, the last generation of the line of big Apple computers meant for the people that need lots of everything, power, speed, memory, inputs/ouputs, etc. built into the machine where it's virtually plug and play. You can simply size it up to whatever you want because it's fully capable of more than anything anyone would use.
My Mac Pro is more the middle of the road Mac, the newer chips but more modest memory (16 GB's) and disks (four 1 TB). It's the input and output that was part of the decision to upgrade from a early 2006 G5 PPC. I run two printers and two scanners and two external disk servers, all of which simply plugged in and worked, and lots of applications simultaneously.
I upgraded to OS-X 10.7 (now 10.7.4), Lion, because many of the applications were being developed and then and now available were built around Lion with the an eye to OS-X 10.8, Mountain Lion. Personally I was happy with OS-X 10.6, Snow Leopard, and it's updates for the look and use, and with the upgrade made Lion do the same.
Now Apple is set to release OS-X 10.8, Mountain Lion, which is built around iCloud and the iPad user features and functions. And that's the "Hmm...", because so far I haven't seen any reason to upgrade to Mountain Lion. Apple is changing everything I like about the Mac Pro and Snow Leopard and Lion to emulate an iPad.
The word stupid comes to mind but then Apple isn't develping Mountain Lion for the Mac Pro but the Apple line of laptop and iMac computers so merge the user experience. The Mac Pro is more an after thought to fit Mountain Lion into a bigger computer which users won't need or want.
I didn't upgrade to Lion for about 3 months after it's release so it could be sorted out and the software companies could get upgrades or updates to the users. So I'll likely do the same but it may even be longer, like 6 months or more because all that I've read about Mountain Lion makes me think that, for me, it's not worth it, yet.
I don't use the iCloud except users had to move their Mobile Me account to it or lose it. I only keep my e-mail in iCloud and have no interest or intention of doing anything more with iCloud. It's why I have the Mac Pro, everything is here and readily available, and I own it. Well, except one application, Adobe's Muse, which is leased and I'm testing for a year.
So to me the only reason to upgrade is so I can turn all the iCloud and iPad features off and keep the Lion user interface, features, functions and use. That means all I would gain is the stuff "under the hood", like application certification, more sandboxing applications, security, etc.
Is that worth it? That's what I haven't decided and why it's a "Hmmm..."