I like my 12" Powerbook. My mom bought it for me on educational discount (which, sadly, isn't that much). Honestly, had I been in the market for something with a bigger footprint, I would not have gotten the 15" model. I probably would have gone for a Dell 600M with the coupon code I had when I was pricing these out.
The Mac is easily be most mobile machine I have ever seen or laid hands on. It goes to and comes out of sleep in less than a second - no matter how long it's been asleep for. If I turn off the Bluetooth and Airport adapters, it'll sleep for days on a mostly full battery charge. Really nice, since I never have to reboot. Cruising for Wifi hotspots at Logan Airport (when I was on a business trip a couple weeks ago) was way easier than my traveling buddy from work who had one of work's loaner (windows) laptops.
Battery life has been outstanding.
Software wise... my major disappointment thus far has only been the fact that there's not a Linux driver for the Airport Extreme card. Given the lack of PCMCIA slots on the 12" models... I decided not to try Linux on here. At least not yet. I haven't had motivation to move to Tiger, but maybe when I do, I'll repartition and dedicate some space to a Linux install. Yeah, some of Apple's built in apps really blow (iChat being one that comes to mind) but VersionTracker.com is a great tool for getting the apps you need. Contrary to popular belief, there is plenty of Open Source and Freeware stuff floating around to do what you need. I installed Portage for PPC-MacOS on here as well, which isn't altogether too useful yet but it's getting there. Definitely install the X11 user and dev libs on it when you get it, first thing, if you get one. One more thing - iPhoto kicks tail. One unified interface for grabbin' your pics, no drivers or cheesy grabber software to install, no matter what kind of camera they come off of.... definitely an awesome way to interface with your cam.
Have I regretted switching to the Apple? Yeah, at times there's been stuff that I've wanted to do that is pretty straightforward on Windows that's bass ackwards on OS X. But then I grab my machine and head to the coffee shop.... and I just throw the Bluetooth wireless mouse in a pocket and grab the laptop itself - no power adapters, no expansion cards, nothing the plug in - and I've got several good hours of power surfing to do while sipping my favorite latte. And then I don't regret getting the Mac. It's mobility is incredible. A lot of those things would hold true with the 15" model as well....
However, one thing I didn't buy the laptop to do was play games. I loaded Wolfenstein ET, just for giggles, but haven't really tried to play it yet at all (can't find anyone to play with). All my games are Windows games and when I want to play... I sit down at my full-power desktop machine. I find myself using that thing less and less though, bewteen the laptop and the HTPC machine.