Yup, I've just switched from a MacBook Pro 15" 2.6 classic to a 13" 2.53. It wasn't through choice though. The 15"'s mother board went belly up, which after 15 years of Macs from Classic to Quadras and Powerbook G4 (Which I still use for emergencies thankfully), is the first time I've ever had a hardware problem. The 15" (using Tiger) was only two years old and my faith in Macs took a huge knock.
However, I'm awed by the 13". It is like quicksilver and considerably faster than the 15". I run my business from the laptop, which is on most of the day although I have no heavy software usage apart from occasional daily Photoshop work. I also decided to do away with Firefox and Entourage and try Mail and Safari - my thinking that Mac's own browser and email program might make the computer more stable. I got lots of Entourage hang ups on the 15". Still recovering from the additional expense of a new Mac.
The distributor neglected to tell me there's an extended warranty on the 15"'s motherboard, which I was very pissed about. Still fighting with them over it but I doubt, it will go anywhere. Generally upset with distributor's customer backup. All these glitzy showrooms and staff that don't have the product knowledge to match. Nice people but buskers.
I am beginning to fall in love with the little 13". Phew - it's fast!