I just bought a powerbook 15"
and I would definitely recommend the purchase. Here's some of my subjective evaluations.
Speed:
I don't notice any difference between the PB and my Dell in 'normal' operation. Surfing the web, editing files, checking email, Microsoft Office all run well. I think you'll notice the difference if you use programs that really require significant processor work. When running processor intensive scripts in Matlab and Mathematica, I find the PB runs like a (hypothetical) 1.5 GHz Centrino (and quite a bit slower than my P4 Dell). I don't do video, but I'd suspect you would find a similar situation in that realm.
One thing the PB is MUCH better at is multi-tasking. I can have 8-10 active apps and the machine is just as responsive as having one open, whereas on my Dell, the machine justs slowly grinds down as I add more open applications.
Feel:
This is by far the best laptop I've ever worked on. In particular, this is the first laptop with an excellent keyboard. I usually have to plug in a keyboard to feel comfortable, but not on the PB. The scrolling trackpad is another great thing. I actually usually take my hand off the mouse in order to use the scrollpad for scrolling.
The screen is excellent, although I will say, that the newest Toshiba Satellites have an even better screen.
Overall this thing looks and feels like a million bucks!
Connectivity:
Plug and play, with absolutely no hassle on LAN networks and Wireless networks.
OS:
This is pretty much why I made the switch. Tiger is an excellent place to work. Installing and uninstalling programs is a snap. Unlike most other Mac users, I really like the Finder as compared to Windows Explorer. The biggest thing is the complete configurability of the system. You can build shortcuts to do just about anything, and there are already alot of smart shortcuts built in. The drag and drop ability with just about object on the desktop is a big plus as well
UNIX:
I don't know if you will be going here, but if you need to spend significant time working in a Unix environment, X11 is much better and cheaper than PC solutions like Reflection X and Cygwin.
Applications:
Most major productivity applications (Adobe PS, Office, Firefox, etc.) are available. Specialty programs like Pro-Engineer or Solidworks may not be. If you have specific application needs, I'd strongly advise you to look before you leap.
Finally,
If you can find the software you need, then you'll probably be really happy with a Powerbook.