Currently, operating the timeline is my biggest woe, though I suspect this may stem from the complexity of the sprites I'm working with.
There was significant lag between whenever I was making changes in the main window and when then proceeding to click & drag to navigate the timeline. My initial clicks sometimes do not register, and pushing the time line too fast when trying to get it to read my mouse clicks leads to 90% of my crashes. Thus far I've alleviated this by patiently waiting between each edit and when trying to navigate the timeline, even then it seems to hit a few spikes that cause it to choke and give up entirely, crashing.
Running windows 7, 64 bit, on a fairly beefy alienware (though I'll also be using this on mac). Not sure if this is more of a driver / memory thing or if I'm just pushing the beta version too much.
Good luck.