I've had luck installing most Ruby gems used by my projects. The only exception so far has been Nokogiri, and even in that case there were instructions on the project's web page that got it to install without a hitch.
I've been using it for a bit. There are a couple of snags, but nothing has stopped me from getting work done. Most of my issues have been with homebrew formulas that aren't updated to work with 10.10 (mongodb is the one on the top of my head). Other than that, the only real issue I've had is with installs. Each update has taken upwards of an hour and a half to install while it constantly tells me 4 minutes remaining. Post install, pretty smooth though.
Things I'm regularly using without issue: PyCharm, Thunderbird, Chrome, Firefox, Subline Text 2, and iTerm
Using it on a mid-2010 MacBook Pro, for about a week. OS X itself is stable.
However, WiFi with unreliable internet connection (like tethering over a phone) is very flaky – the WiFi connection itself keeps dropping.
Safari has trouble with some video streaming sites – YouTube at 1080p, Netflix. This can be because of my older hardware, but never had any issues with Mavericks. Also WebGL does not seem very performant. Chrome does all of this perfectly fine, though.
If you use virtualisation with VMWare Fusion, note you need 6.0.4 for it to work under Yosemite. 6.0.3 will not run.
The new dark mode menu bar and dock are nice, but there are some rendering issues. It seems 3rd party menu bar icons are also not compatible yet, so most will be invisible or broken in dark mode.
Been running it since beta 2 or so. Latest build is pretty good. Wifi drops out every once in a while and something weird is going on with my audio routing where it keeps trying to route to airplay devices that may or may not be present. VMware fusion works now if you use the technology preview release.
I use it full time for iOS dev and casual use and haven't had any showstoppers. I have my old mavericks install in a second partition and rarely reboot into it.
Otherwise it's in good shape for a beta OS that's still a couple of months from GM.
I am running beta 4 (14A298i) on a mid-2013 11" MacBook Air. It sometimes loses the WiFi connection, but reconnects automatically. I've noticed the laptop will restart itself with PowerNap turned on, and plugged into an external display and external drive. Turning off PowerNap seems to fix it. Everything works pretty well. Handoff is a little funky, but you'll probably not be running iOS 8.
Using the developer preview for about a week. I have a 2012 Air 13". DP3 was mostly ok, but would freeze and immediately reboot once or twice times a day. The newly-released DP4 seems stable enough though.
There's also something weird about Safari on Yosemite. Sometimes the tab bar would just hide completely and to get it back you'd have to reopen Safari. (The tab switcher and cmd-{ cmd-} would still work.)
I use it daily : Emacs, Illustrator and Photoshop works quite well; Safari has gone a long way and is now a pretty cool browser (except it crashes often, but I guess this is what betas are for). I had no problems with my homebrew packages. Indeed I've just realized that I've installed a dual-boot Mavericks/Yosemite but I'm no longer using Mavericks.
I'm using it on some older hardware (2011 MacBook Pro) and it's been working just fine for me. It's maybe a bit more sluggish than Mountain Lion on the same hardware, but the difference is small enough that that might just be in my head.
It has been fine for me. I've been using things like Xcode, sublime text, mamp, codekit, git, mail. Haven't used virtual machines much though so can't comment on that. I typically find the OS X betas easier to work with than iOS
I had issues with installing Photoshop and Lightroom through Creative Cloud in Beta 3, but it looks like Adobe fixed those issues, and everything's been fine since.
And Photoshop CC and Lightroom 5.4 have been rock-solid for me.
