I tend to be a late adopter of operating systems. I went from Windows 95 to XP to 7, skipping Vista, Me, and a number of other dogs whose names I can't even remember. I would still be running XP except that when my son switched from Linux to Windows 7 I sat up and took notice.
When I switched back to the Mac we were on Leopard. I skipped Snow Leopard, upgraded to Lion, and skipped Mountain Lion.
I need a pretty good reason to upgrade.
Add to that: I have a 2013 MacBook Air with 4GB of RAM on which I routinely run Word, Chrome (typically with about a dozen tabs open), Mail, Calendar, Numbers, Evernote... and VMWare Fusion, with a Windows 7 VM running. Don't get me wrong, I love my Air, but it has seemed a liiiitle slllooow at times, e.g. booting up or suspending the VM and I've been kind of coveting the 8GB Air. And for a while I've been suspending the VM to try to keep things running better. So new OSes tend to have a bigger footprint and be a little more bloated, and I was afraid Mavericks would push me over a tipping point.
On the other hand, I've been using two monitors for a number of months now, and the fact that you can't switch them independently seemed entirely brain-dead to me. So Mavericks was supposed to do something about this, and I was very interested in that.
So I looked on line and a few people were complaining about it being slower... I asked around and nobody seemed to know much about it, until my buddy Tom told me he'd upgraded and his systems were actually running faster.
So over the Thanksgiving weekend, I decided to go for it. I've been very pleasantly surprised: things are noticeably faster and it has breathed new life into my 4GB Air.
But of course, there are pluses and minuses...
1. I love the multiple monitor support; it is making my life a lot easier. Rather than each "space" having two screens, you can create independent spaces on each monitor, and switch the monitors independently. This is heaven. Oddly, though, you seem to lose the ability to assign apps to individual spaces, it appears that you can just assign them to monitors. Oh, except that when you unplug the external display, all of a sudden you can assign apps to specific spaces, which persists when you plug the external display back in. (And, BTW, the spaces from the second display are assigned to Display 1 when Display 2 is unplugged and are re-assigned to Display 2... the way it should). But of course, there needs to be something bizarre: I have 6 spaces on Display 1, and 3 spaces on Display 2... but when I unplug Display 2 I somehow have only 8 spaces total, so there are still a few things to shake down on this cruise.
2. The first big problem I had was with Mail.app. All of a sudden I had 18 unread messages that would not appear in my supposed "smart" inbox. Eventually found a solution; I don't think I used this post but it was equivalent: https://discussions.apple.com/thread/5477662. You have to trash a bunch of files in your Library (I just trashed the EnvelopeIndex files, which did the trick... this particular post has you trash a few more). Once it rebuilt the indexes, I could find the 18 "unread" messages which turned out to have been read years ago, but at least I could now flag them as read. Next freak-out: my flagged messages were all gone, and this would have been a fairly serious problem for my life. Fortunately, they reappeared a few hours later. Or maybe it took overnight. But then again, I have 175,000 emails, apparently. Your mileage is likely to be better. By a lot.
3. The next problem: VMWare Fusion 4 told me there wasn't enough physical memory to start the VM. Ouch! But I quit a few apps, started the VM and it worked... and when I restarted the other apps, everything worked just fine and seemed faster than it was before. But then, it seemed to have problems connecting to the network. The network connection had been a little persnickety in the past (like, had to use NAT rather than bridged, or maybe the other way around). I love VMWare Fusion, so I bit the bullet and got VMWare 6 for $42. Networking works, things run faster... and with VMWare 6 I am not getting the "not enough physical memory" message.
4. I did the iWork '09 upgrade. Not so happy with that. It lost the autofill in tables, which I use a lot... and it broke scripting. Back to Numbers '09 for me. But there was some panic when my scripting files seemed to be corrupted. I tried reloading them from Time Machine and those seemed to be corrupted, too. I eventually figured out that the problem is that it forgets about the Numbers '09 Applescript dictionary. You need to quit Numbers 3, restart Numbers '09, then load the script. Maybe restart for good measure. Honestly, maybe skip the iWork upgrade. Although I kind of like the cleaner UI design.
5. My script for the Calendar works, except the the last step is suddenly veeerrrry sllloooowwww, like 30 seconds. No idea why. Very annoying. I like the new Calendar UI.
6. A couple more things: I used Time Machine and Carbon Copy Cloner to make my backup, but discovered that my new WD myPassport 1 TB external drive, which I really like, is formatted with MBR partitioning, so it can't be bootable, among other things. So: (a) every time you get a new hard drive for your Mac, don't just repartition it like I did, reformat it using GPT, and then partition. (b) CCC now costs money, SuperDuper looks like it is a more inexpensive solution to do the same thing... which is mostly irrelevant now, with Time Machine, though having a bootable backup looks like it could come in handy.
7. Oh, and somewhere in there my SSD went from 10 GB free space to 30 GB free space!
So: faster, better monitor support, and more free space on my SSD! Overall looks like a big win so far!
Update 8 December: still very happy with Mavericks... it (together I suppose with the VMWare Fusion upgrade to 6) is enough faster that I have gone back to leaving the Windows 7 VM running all the time rather than suspending it when I'm not using it, which is definitely more convenient. At the same time, browsing is noticeably faster, whether it's in Chrome or Safari.