I installed the Windows 7 RC on my laptop some time ago, and the only problems/annoyances I ran into so far are some older games and programs that don't work anymore (like with any other OS upgrade, especially when it's not final yet), and the inability to install unsigned drivers without changing boot options and removing some watermarks from the desktop.
Also, like in Vista, I'm missing the option to remove the Ease of Access button from the logon screen (Me and my housemates like to mess with each others' laptops from time to time). For now I disabled it using a 3rd party tool. It's not like this is a huge problem though
Overall, I think it's a huge improvement over Vista, but it lacks the "freedom" XP had compared to Vista (drivers story amongst others). I guess this is part of the improved security it's supposed to have.
EDIT: Also, I can finally use the 4GB of ram I paid for when buying this laptop. It came with 32bit Vista, which I downgraded to XP right away.
/ 
