A few weeks ago we purchased a copy of this and didnt have all the programs closed that we should have before downloading the updates and we locked it all up and it ended rather badly with a complete reinstall from time machine being required.  the only thing we learnt is just how incredibly easy it is to restore from backup using time machine; a piece of cake.  we thought the microsoft code was as buggy as the jungle at the time, but in retrospect I think we were probably just a bit unlucky (& careless!).  the conclusion is we were moving up from Office for Mac 2008 and it is significantly better, in particular it opens files much more quickly on my fairly standard 3 year old plus black polycarb mac book.  If you wondering whether it is worth the coin i think it is.  the user interface is also significantly improved with it being much easier to find the function you are looking for, which is important on a 13" screen.