for leisure use it's fine, so long as all the drivers meet WHQL. For serious business use/line of business apps, a lot of testing needs to be done to ensure 100% compatibility, otherwise it can be crippling.
SP1 is due later this year - I think that will bring the stability business users seek, though we use it in anger within Microsoft and it does work really well in our environment.
Office 2007 is bloody fantastic - I recommend anyone running 2003 to upgrade. There's so much more flexibility (though the document/spreadsheet formats are all new).