Technology changes at lightning speed, it is a fact of modern life. I remember once seeing it written that if automobile technology evolved as fast as computer technology did, then we could all be driving flying cars that get a million miles to the gallon, and we would throw them away when we reached our destination because it would be cheaper to buy a new car for the return trip home than to pay to park the old one.
Or something along those lines.
Anyway, the point is this: If I decide that I NEED a computer today, I will shop around and buy one that does what I need, at a dollar amount I am happy with. A few months down the line, that same dollar amount will buy substantially more power. This is a GIVEN. But that doesn't change the fact that (a) the machine I bought is STILL powerful enough to do what I bought it to do (it did not magically become slower), and (b) regardless of current market value, I was STILL happy to pay that dollar amount when I bought the machine. And, the difference between what you paid then, and what you would pay today, is the cost of all the useful work your computer has already done between then and now!
Unless your needs have dramatically changed (and Microsoft and the gaming industry appear to promote this by constantly releasing new software that "requires" faster and faster machines), your computer is still doing what you paid for it to do. You will "get your money's worth" (as Zoran stated it) as long as your computer continues to do what you bought it for.
If you are seriously upset that you "should have waited" or you are jealous that your buddy paid less than you did, I submit that you are either being juvenile, or you didn't really "need" the computer you bought in the first place.
I have a 12" Powerbook, and I paid over $3000 Cdn for it. I am SO EXCITED that sometime this year for well under $3000 I could buy a 15" MacBookPro which is going to blow my laptop's performance out of the water. But do I NEED to? ... no, not really. My 12" is doing what I need it to do, and until that changes, I am still getting my money's worth.