Yes, Logic 9.1 is working well here. Also when you update the Logic 9.0.2 application remains in your application folder so you can revert to that if you wish to.
Logic 9.1 has not bitten me yet, but you will need to make sure your 3rd party plugins are 64 bit compatible to run Logic 9.1 in 64-bit mode. Steve H has made some free videos on using Logic 9.1 - 64-bit mode.
In fact you can run Logic 9.1 in 32-bit or 64-bit modes!
Have a look here:
Also, there are some restrictions if you run it in 64-bit:
This is the original thread on Logic 9.1 here:
and also the Apple's support document:
But first watch the free Logic 64-bit videos for lots of cool information!
Stai bene :)