Is Git now the only way to install Drush?
Drush moved to GitHub a few weeks ago.  The official home is now: https://github.com/drush-ops/drush

The current preferred method is to install with composer:

composer global require drush/drush:6.*


If you are working with Drupal 8, or need a patch that hasn't been backported, then you can work with the latest version:

composer global require drush/drush:dev-master


You can also use composer to install a particular version. 

You can install it manually; and normal GitHub retrievals apply.  You can download the master (link is on the main page), or clone the repo locally.  When you clone locally, you can checkout any tag you want.

I have it installed in my home directory, eg

cd ~
git clone https://github.com/drush-ops/drush.git drush


After, you need to run composer 

cd ~/drush
composer install


I then put ~/drush/ in my $PATH:

export PS1=$PATH:~/drush/


Everything should work as normal.

Sidenote.  Even if you don't want to contribute back, tracking your whole site with git is a great way to handle dev/live, and for backing out oopsies.