Office 2008 can be slow. I see the slowness on the first load of an application. Word is the worst for me. Once its bee loaded, closing it and restarting if much faster. That said, iWork is faster.
I have had no crashes at all. Powerpoint once fully loaded is really fast for me, I see zero lag.
The BIGGEST thing that causes problems is left over MS-Office 2004 crap stuffed in your libraries folder. Remove Office 2004 via the 2004 uninstall, search for Office 2004 crap (download a eval of PathFinder, its search will find anything). If the install of 2008 finds stuff from 2004....have it remove it. I cant stress this enough, its all over the forums at MS.
My basic opinion is that if you need 100% compatibility with Office docs on a PC for work or school, then get the student version. If you need Exchange support, then get the standard version. I think Entrouage is a horrible application, and it changed the least. Outlook 2007 on the PC is far, far better email application. Mail/addressbook/iCal is also way better.
I use Office 2008 when working with stuff that needs office compatibility, and iWork the rest of the time.