Adobe CC 2015 upgrade asking for OS 10.9 or higher.

Upgrade to 10.10 or don't upgrade CC would seem to be your choices.

10.10 is faster and more stable than 10.9, no idea why you want to stay out of date.

You can find horrible ratings on any OS ever released, Mac or Windows. It's also a known fact in the business world that dissatisfied customers are much more likely to make their feelings known than those who are not. There isn't a single release of the Mac OS or its updates that didn't generate numerous posts here of how bad "OS xxx" is.

I've been using Yosemite since 10.10.1. It is much faster than Mavericks, or any other OS before it. You do need at least 4 GB of RAM for reasonable performance. 8 GB or more would be preferable. The only thing to get used to, more than anything else, is the overall change of the desktop look. After using iOS 7 for quite some time on my iPad (and now iOS 8), it wasn't a big deal to me to see Yosemite with the same look. Nor was it a surprise.

Some people can't do much except complain, it doesn't matter which version of OS X, they always complain.

Rule numero uno before upgrading. Make a backup of your current drive.

I'll give that a +