I thought I'd jump in early(ish) with the GM version of Yosemite.

Unfortunately, it is not an easy journey yet so here are some infos collated from around the web about how to get around the various issues I encountered

For comparison with your system, I'm running a MacBook Pro Early 2011 with 16GB RAM and SSD drive, with my old spindle drive in place of the DVD drive. I'd previously performed all updates and was running XCode 6.0.1

1) Extremely slow install

Since I was doing an upgrade, this might not be the same for everyone, but my installer got stuck at "Less than a minute remaining" and stayed there for 24 hours!!

A few forum threads noted that earlier releases had an issue if you have a very large /usr/local/ directory (e.g. if you have Homebrew or MacTex installed there, and you should just wait it out. So I waited............

For me this wasn't actually the case - it was just stuck at the end of the installer. So after 28 hours I gave up and powered off my machine. Yosemite loaded up in all its login-screen-gaussian blur wonderment.

Success!

Immediately following login I was prompted to upgrade my Command Line tools by the App Store. That was a very quick update, so that was good!

Next I checked my Ruby version. I'm using RVM and the Ruby installs seemed to have come across OK. However as is usually the case lots of linked libraries don't work for example OpenSSL etc, so we're gonna have to recompile everything

2) The fun begins

Started off by updating RVM

rvm get head

Then I tried installing a fresh Ruby (most of our apps are still 1.9.3)

rvm install ruby-1.9.3

Basically if failed after trying to load command line tools saying I had to accept the XCode license.

Off to XCode then...loaded XCode and accepted the updated license agreement, then ran the rvm install command again

3) TL;DR; GCC Homebrew install is broken

Since this is a prerelease copy, Homebrew does not yet properly support the upstream changes for GCC. So first I update homebrew, to see if that helps.

brew update

Now trying again with the rvm install I get a bit further - more dependencies install for GCC but when GCC installs we get a failure . Writing this after the fact, I can't show the error right now but it basically is related to this ticket: https://github.com/Homebrew/homebrew/issues/29845

Following all the steps in the ticket, I found the only solution was to wipe out my whole Homebrew install and start again.

3.1) XCode Update

Actually at this point I figured it was worth living right on the edge so I grabbed a fresh copy of XCode 6.1 GM Seed from the Apple Developer site and installed that. Pretty slow install.

3.2) Homebrew rebuild

Now back to re-installing Homebrew...

rm -rf /usr/local/Cellar /usr/local/.git && brew cleanup
ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
brew doctor

brew doctor told me to update my XQuartz version to 2.7.7

http://xquartz.macosforge.org/downloads/SL/XQuartz-2.7.7.dmg

Install that, then apply the GCC patch...

brew edit gcc

After osmajor section

# tobinjones patch gcc https://github.com/Homebrew/homebrew/issues/29845
patch do
  url "https://gcc.gnu.org/bugzilla/attachment.cgi?id=33180"
  sha1 "def0cb036a255175db86f106e2bb9dd66d19b702"
end

Now re-install GCC. This can take a loooong time.

brew install gcc 

4) What Next?

So, this is how far I am now. Will keep this updated as we make further progress