The beta itself pretty much represents what the final version will be like except for one thing: the toolbar. There is no custom toolbar in 3.79 because I have to wait for the author of DMU.DLL to release a 6.1x compatible version. This will probably take another few weeks since he doesn't have access to his source code at the moment.
Other than that, it should be fairly bug-free since it has already been tested for quite some time.
Even if you will miss the toolbar, I recommend you to update to mIRC 6.12 with NoNameScript 3.79 if you want to avoid being exploited.
Keep in mind this is a beta. You can report bugs in our forum.