Win7 boots quicker than Vista, it uses the hard drive less, and IMO looks rubbish next to Vista, Performance wise, its on par with XP. Compatibility wise, XP runs everything I have, Vista less so, and Win 7 even less. My mobile broadband modem is detected immediately first time every time in XP, every second time with Vista and every third or fourth or after a re-boot in Win 7. Security, well sure its more secure, but so was XP compared to 98, we'll see where 7 is security-wise in 5 years.
Me? i use win 7 because of Mediacentre. Thats all.