I've been using it for a couple days now. If you are editing your homescreen, stability is not great. About 5 FC's today while editing my theme. Also, the more time you spend editing the slower the editing interface gets. If you are just using the app though stability is good. Haven't had any FC's while just using it. It has not affected the speed or stability of other apps.
Basically, they just need to work on stability when editing your theme.