I recently used the tool to upgrade my 64 bit mint 6 to mint 7.
I had two complete freezes/lockups after the upgrade. The first happened about 10min after gdm login, the second during shutdown. After those, I've had no problems whatsoever. This was on a T61 laptop with very little tweaks on the mint 6 install.
So yes, upgrading works if you are ready for a few glitches and I think I'll use it in the future also, since I'm too lazy to re-install my favorite apps
ps. my first post on this forum, I switched from Ubuntu to Mint somewhere in January. Love it