Call me a skeptic and call me cynical but I've been deceived so many times that Apple has fixed things that I don't believe anyone else. It's my belief that by 10.x.4, the version of the operating system is finally stable.
Mac OS X 10.3.4 finally had a decent amount of functionality and stability that I could abandon Mac OS 9.x, for instance. 10.4.4 and 10.5.4 signalled some great stability, although 10.5.x was a step backward for PowerPC-based machines. The speed and stability was in 10.4.10.
I pretty much considered Leopard (10.5.x) a disaster. It improved the pathetic performance on Intel-based machines to fair, while ruining it for PowerPC-based machines. It was about this time that Avie Tevanian left the company. If he was frustrated with Scott Forstall, I could understand that.
Snow Leopard (10.6.x) for Intel-based machines only, was a step in the right direction, as it seemed to make one faction happy--the new (often iPhone-related ) Mac buyers, who had no legacy equipment. I have a 2009 MacBook that works pretty well with it, especially compared to my hobbled late 2004 PowerBook on Leopard.
Blah blah blah...I'm trying to establish some background but I write too much that may not interest anyone. Sorry!
Anyway, Apple developed Lion to be a little this, a little that and Mountain Lion to improve on it.
I'm still a little uncomfortable with Mountain Lion because as soon as I connect my trackball the interface changes and some applications have a dual mode to deal with the new scrolling and scroll bars, etc.
My number one gripe with Mountain Lion is the delay. I've got a 2.6 GHz i7 processor with 16GB of RAM and a 480 GB SSD. It's nearly as fast as you can go--and there have been huge delays from 10.8.0 to 10.8.3. There have been delays starting applications and delays quitting them. 10.8.4 seems to erase the delays. Update 2013.06.10: talking to a buddy with a MacBook Pro with nVidia 320M graphics hardware, there is no noticeable enhancement.
I'm interested to see how it handles when I have 1000 photos to transfer from a CompactFlash or SD card and then, attempt to process them. Usually, the fans start running and the temperature rises to near 100 degrees C--too hot--unless it's iStat Menus that can't count. That was with the cooling pad running beneath the machine. I switched to a 3 fan cooler from Targus and it helps a bit more.
I suspect that this is next to the last update for 10.8, since WWDC is next week. I wish they'd fix iOS 6.x because I hate seeing notifications that show up 13 hours later. Instead, they'll foist an additional set of problems upon us, and those who can't upgrade will be lost without security updates even. I'm not being cynical--that's just history.
Update 2013.09.03: Geez, waiting for an update to anything, despite security issues, can be hell. It's been about three months, and I see weekly reports about 10.8.5 but nothing. There have been a couple of security updates, but nothing much to fix bugs. I've been finding more and more than the 2012 MacBook Pro and 10.8.4 won't work with my USB thumb drives. There seem to be conflicts with USB drivers that weren't there before the 10.8.4 update. As a backup, I tried the same thumb drives with 10.6.8 on the 2009 MacBook and it worked just as well as it did previously. I just need to make the data available to multiple machines.
It shouldn't be a surprise that an operating system fix breaks as many things as it fixes. In any case, September 10th is coming, and there is an expected announcement of the new iPhone models, but computers (other than MacBook Air, already updated) should be updated and operating systems should see updates. There will likely be iOS 7.0, as there will be new devices, but (Mac) OS X 10.9 should be available also, and possibly 10.8.5. I certainly need fixes.
Update 2013.11.25: Time has come and gone. 10.8.5 and 10.9 and iOS 7.0.4 are all out. Updated devices came along and 10.9 will help immensely with battery life. 10.8.5 will help with overall stability.
I've been considering a change to 10.9.x but I want to wait until it's fairly stable, especially when doing such a large download. I might put it on the 2009 MacBook. It's supposed to be a bit more conservative overall, so it might work better than 10.6.8, though I have my doubts. I'd rather test it on a not so critical machine than to upset my workflow, even though Mountain Lion could use some fixing.