Well, I used Linux Mint 13--the current long-term support release--with MATE and I can say that it is pretty much rock solid and I didn't need to tweak much to get it working as I wanted to. I've switched to Cinnamon now, as I liked the latest release of that better than MATE. If you do go for Linux Mint 13 with MATE, I'd recommend you enable the backports repository, as detailed here http://blog.linuxmint.com/?p=2225
, which will give you a newer version of MATE that is much improved. Or you could use Linux Mint 14, but I guess you want the long-term support release
You can download either from there: http://www.linuxmint.com/oldreleases.php
There are some things you need to consider; MATE isn't GNOME 2. It is a fork of the last GNOME 2 release, on which the MATE developers have worked to fix bugs that were still in GNOME 2, worked to include new functions and features, and most importantly for you have worked to rename all the programs and libraries so none of those conflict with GNOME 3 (as GNOME 3 largely uses the same program and library names and such as GNOME 2). That means you will have a small learning curve, and some third party programs may need a bit of fiddling on MATE to get working just like they used to on GNOME 2. But with MATE being the second most popular desktop environment with Linux Mint users, and it having been around for almost a year and a half now, the chances are good that if you have an issue there are other users here that can help you.
A lot of users of Linux Mint started out with Ubuntu. The good news is that anything that is available on Ubuntu is available on Linux Mint, so you won't be throwing away any knowledge. But you are running a three year old Ubuntu version. No matter whether you stay on Ubuntu or move to Linux Mint, if you install a newer operating system you will also get newer versions of your applications--and those may work a little different perhaps (with new or changed functionality). So there may be another small learning curve there, but that is the price you pay for running a long-term support release. Yay stability, nay latest functionality.
Lastly, if you are a Compiz addict you should know that Compiz isn't preinstalled on Linux Mint with MATE, but there are good guides available for how to install and configure it.