I have a MBP from 2007 and an Acer Aspire netbook from 2007, both running relatively strong and I've taken both around the world in a backpack several times (often together).
What's gone wrong with them?
MBP - The case is full of dents, and the screen is starting to get dark. It's balls slow these days and Apple really doesn't want to support it anymore.
Netbook - it can't detect how much battery is left anymore, but otherwise seems fine. It's also balls slow, and there's not really a way to support it.
All that being said, replacing my netbook with something similarly priced will cost me $300-400 and get me a hell of a lot better piece of hardware. Replacing my MBP is going to cost me a pretty penny that I'm not looking forward to.
Note: I actually did "replace" my MBP with a Dell Studio a couple of years ago for about a thousand bucks and got a ridiculous amount of hardware for that price. It still looks almost new, but it "feels" cheap and fragile, even if it hasn't actually shown any wear and tear. My MBP on the other hand feels incredible, but has collected so many battle scars from fairly undramatic handling that I wonder if aluminum really is a good case material in the long run. The metal around one of the ports in the back is so buckled the port is no longer usable.
Note 2: It's interesting, being an East Coastie, that when I see MBPs it's usually young professionals at Starbucks or some such putting together business consulting slide decks or something. It's mildly unusual to see developer types out here with Macs (or see them with Macs not running Windows). I'm sure it's an effect of banking and government on the ecosystem. But it's almost always non-tech types using Macs here...usually as a response to having crappy corporate Windows XP/7 systems forced on them in their cubes at work.
Note 3: I'm not really a developer type, so personally I think Macs are "OK". I can see why West Coastie Web Startups use them. The Unixland is decent enough, the GUI is pretty good etc. For folks who want to do that kind of stuff the ecosystem is pretty nice.
For me, most of the stuff I want to do with a computer either isn't as good, doesn't exist, or is a second class citizen in the Mac world (it's mostly music composition, retrogaming, other entertainment) but is miles ahead in maturity and choices in the PC world. Portable software practices make lots of that go away when I'm stuck on my Mac and want to have some fun, but it still feels less mature than the Windows counterparts.
For business, almost none of my customers are using Macs, so I'm back into Windows land again.
It's frustrating, I'll sit down to really work with my Mac for a bit (and did for a few solid years), but find myself inevitably drawn back into Windows world to "just get shit done" in many cases.
Running Windows on an expensive MBP seems silly as the hardware seems to be about as durable, but the Windows machines are virtually disposable they're so cheap...and I couldn't care less about the feel of the machine.
I think changing work environments to a different kind of shop would probably offer me a better opportunity to stick with OS X, it's a kind of network effect.
But outside of some personal frustrations, I can see why either ecosystem would be a perfectly valid choice.