I have to weigh in (ha! a pun!) whenever hardware is discussed. The two situations Lora and TDavid talk about are distinct and there is room in the world, obviously, for both of them. Who buys all the shiny new gadgets that hardware OEMs produce on such a regular basis? Who can afford to buy all the shiny new gadgets? I don't know anybody who bought all of the Motion series of Tablets from 1100 through 1600+800 out of their personal funds. Who does?
I like to take a middle ground in the debate.
First and foremost, let me admit to my Hardware Junkie nature and tell you that the first thing I think about when I see some shiny new toy reviewed is "I want it". I always want it. You cannot tell me that you, dear reader, do not feel similarly when your [favorite genre of stuff] is featured in a media piece. Because you do.
Second, let me tell you that I have quite the inventory of spare parts, so quite often for myself, the decision is not "new or used" but "what can I buy to rebuild my system better/faster/stronger".
However, being that I am of limited resources and that I have a young family to raise and care for, I have learned to let the feeling wash over me and it passes. Sometimes, like at Christmas time, I get carried away, but it's never to the tune of 60" DLP HDTVs or new Toshiba M4s with all the trimmings. Some people don't have limited resources (like this guy or this guy), but 99.999% of us do have to live within a budget. Electronics also have a high infant-mortality rate, but thereafter a very low mortality rate. So most electronics, once they're known to be working, stay working.
But some things don't make sense to buy used/trailing-edge or piecemeal either. Assembling your own desktop PC used to make a ton of sense and save you a few hundred dollars over buying the latest Compaq Deskpro. Today, the cost of complete systems has come down to less than the cost of purchasing the unassembled parts (sound familiar auto restoration buffs?). It makes sense to build your own only if you want to learn how to do so or if you absolutely must have total control over the parts that go into your system.
And in the case of Mobile PCs (Tablets, laptops, notebooks) how would you roll your own anyway? There's also the crazy uniqueness of the hardware today such that each component has a specific driver available from the OEM's website. Did you know that there are dozens, if not 100+ different varieties of AC97-compatible audio chipsets? Have you ever tried to upgrade a notebook CD-ROM to CD-RW or DVD-ROM?
On top of all that mess, buying a system new (current-new, not trailing-edge-new like TDavid advocates) gets you new software on the box, like Windows Vista (when available). Buying a Motion 1200 today from some liquidation site will only get you Windows XP, and more than likely, XPSP1 rather than XPSP2.
Another apt analogy (alliteration!) is the home-theater-in-a-box decision. Sure, audiophiles and some patient people can piece together top-end systems, but for Joe Sixpack? Most of them are gonna buy that Sony^H^H^H^H er, "generic" box from their local Costco or Walmart, hook it up and enjoy the 7.1 goodness right now without worrying that they're missing 0.27db of S/N or that they have 40 watts per channel instead of 80.
So in sum, I will upgrade or buy trailing-edge gear when it's simple or cost-efficient. The simple (and relatively inexpensive) act of adding a 1GB SoDIMM or replacing a stock 4200rpm 2.5" HD with a 7200rpm one can make a lot of difference to any Mobile PC user.
But, I will buy new gear when I know that it comes with time-saving integration or valuable goodies, like a Media Center PC, or a new version of Windows. After all, the value of sinking my time into piecing the system together, tracking down new drivers and debugging the issues I run into because of integration problems can be quite high.
Budget now, so you can buy new Tablets when Windows Vista comes out!