I finally upgrade my main system to OS X Yosemite. Why didn’t I upgrade it earlier? Because GPGTools, which I use for secure e-mail communications, wasn’t compatible with the new version of Mail. However the great team working on GPGTools released a beta earlier this month of a compatible version of their tools.
I’m happy to report that the beta works quite well (at least as far as my testing is concerned). One thing to keep in my is that the GPGTools team is going to charge for the final release of this updated tool. I have no problem with this because they do excellent work and have committed themselves to keeping the tool set open source. But Thunderbird and the Enigmail plugin are still free, which is something you may want to consider.