I have done it. For me, the biggest failing is that there is no equivalent to the ‘per-recipient’ rules. You either select encrypt to all or not. I need to always remember to whom I am sending and do extra work to encrypt. It also always seems to send the certificate. I have not found a way to turn that off when just signing; it annoys people.
The transition can be easy, but be careful. After you upgrade, before doing anything else, go to Tools->Enigmail Migration from the top toolbar. That will get all the keys and set things up. Because I do not use that toolbar and have it disabled, I did not do this on my first system, and it was painful. Also, backup your TB profile before upgrading. Otherwise, you cannot go back to 68!
Here is a link to a discussion. Here is a FAQ.
Read the FAQ. TB stores the secret keys, and the FAQ seems to suggest using a Master Password. I have not done this yet, because I do not need yet another step to send notes. I am transitioning away from this, so it may not matter to me in the long run.
I have to say, I prefer Enigmail immensely. I find the new implementation wanting to say the least.
Good luck.