Mac for me - a wise department head told me in 1988 to think carefully about what computer I wanted, but he recommended Mac. Brilliant decision, as I confirm every day when I use my company Dell POS (Piece Of S**t). My company is paranoid about network security, but are trialing iPhones with our wonderful friend Lotus Notes.
The MobileMe app with Mac is fantastic - if only BA would upgrade Internet Explorer on the lounge computers, or better, install Firefox. And for that matter, enable the chat feature on G-mail. (Please note Dilly!)