Well, being an old hand at PCs and not quite a complete newbie at Mac, I'll give this a shot.
Powerbooks are exceptionally easy to upgrade, in terms of RAM. There is a small panel on the underside of the case that releases with four small Phillips screws. Once that is gone, there are two SO-DIMM RAM slots, which are very easy to manipulate. Powerbooks come stock with 512 DDR333 PC2700 RAM, and it only populates one of the two slots, so adding another stick is very easy.
Also, RAM for Powerbooks isn't that hard to come by. Popular hardware site NewEgg.com sells an excellent 1GB Kingston RAM module for what amounts to peanuts (135 shipped to Alaska, so around 100 anywhere else in the continental US).
Hard drive and optical drive access is a little tougher - but the excellent website www.pbfixit.com
has step-by-step instructions - for free - with pictures to show anyone who wants to spend the time how to do it. Like all computers, Apple uses standard 2.5" laptop hard drives.
While most people will say "Macs aren't designed for gaming," I have to counter that and say games aren't designed for Mac. Six of one, half-dozen of the other perhaps, but many game makers don't see the point of releasing for such a...limited user base. That being said, the games I personally play are available for Macintosh, and seem to run pretty well. (Homeworld 2, Knights of the Old Republic, StarCraft).
It is true that most newer games just aren't available for Mac. I don't hold any belief that this will change soon, which is why I keep a medium-spec Windows machine hanging around for the gaming tasks.
The price of the Powerbook is something very significant. It's very high, and there's no way of saying that nicer. That being said, I personally believe it's very much worth it.
Firstly, the construction of the Powerbook is top-notch. I'm very happy with mine from that perspective. The brushed aluminum chassis looks and feels very professional - and unlike my 12-pound Sager machine -feels durable, too.
The sheer range of "stuff" that came with my Powerbook made me very happy. Bluetooth, Wi-Fi, an 8x DVD-burner (although admittedly not dual-layer or LightScribe), 80GB 5400RPM drive and the Radeon 9700 more than meet my expectations for what I'm going to use this machine for. The amount of ports available - DVI-I, USB2, Firewire 400 and 800, and the like - also made me feel pretty good about my purchase.
There are also lots of, well, little things that made me feel comfortable dropping 2300 on my Powerbook. The backlit keyboard and ambient light sensing system is a really nice touch that I fell in love with at first shot. The sudden-motion sensor for the HDD is nice too, in case you have a Moment of Clumsy and give your computer a whack it didn't deserve. Battery life is above my expectations (I came from a DTR world, so take this with a grain of salt), as I regularly pull three to three and a half hours of life from my Powerbook.
Least important to me was Mac OS X. Windows has always been nice to me - I wrote a long review with my opinons about that on these forums, so I won't repeat why here - and I've never held much beef with the Mighty Gates. I appreciate all the things that OS X does - it is (currently) immune to spyware and virus problems and I love Expose more than I ought to - but it wasn't a driving force for me. I appreciate that it is a UNIX environment, as a younger IT person I've had little exprience with UNIX, so it's a wonderful learning environment and tool. There isn't - for my time - an easier way to get accustomed to the *nix environs than Mac OS X, which is very much to Apple's credit.
That being said, I've discovered absolutely nothing that I can't do on OSX that I did on Windows. I work in IT and network design, so I have a pretty narrow vision of things that I absolutely MUST have, but OSX has delivered in every facet I've asked it to.
Apple's service thus far has also been spectacular. That's something that is worth more, so much more, than mere dollars to me.
Now that I have my Powerbook, I can't say I'm irritated at the high price. It's an extremely elegant, highly portable, aesthetically pleasing piece of reasonably powerful computer equipment. It does what I want it to with aplomb, and although it won't run, say, Doom 3 at anything like a decent framerate, I don't game much anymore so that is not a consideration to me. It cost me a LOT, but I feel that I have recieved payoff in excess of the price of admission.
The Powerbook isn't perfect for everyone, certainly. I've heard good things from people who do photo editing or rendering on the Powerbook, though these are well outside my realf of experience. High-end gamers are served much better by high-spec gaming machines or desktop-replacements like Sager or Alienware.
As always, your mileage may vary - these are my reasons for picking up a Powerbook, since you asked.