I've got MT 2.64 installed on my site and I was curious if the upgrade to 2.661 was vital in some way?
Is there a big difference or improvement with the upgrade?
The 2.661 has features that will help you control comment spamming. That is the main difference. You can control a lot of it under 2.64 by using MT-Blacklist.
If everything is working well for you you might want to wait until MT 3.0 is released.