I agree wholeheartedly! The best deal is where if you were to plot the relative performance vs. price, the unit just before the slope goes up exponentially. This tends to occur in the following places:
1. In a line where just clock speed goes up and all units are rather expensive, the second to the bottom is usually a good choice. They tend to be on sale frequently as everybody will buy the cheapest one and the most expensive one, so you get a better deal than usual.
2. In a line of products where there's a natural performance break (such as the ATi x16xx -> x19xx series boundary) the cheapest higher-end product makes the most sense as it's usually not that much more than the highest lower-end part and will perform a lot better than it.
3. Used-to-be high-end parts right after the introduction of a new part. For example, if you had wanted a Pentium D to put in your existing 945 or 955 motherboard, they got a ton cheaper after the Core 2 Duo debuted- prices dropped by at least half at the upper range.
4. In a line that goes straight from budget to mainstream as far as prices, the lowest mainstream part will generally not be that much more than the best budget part but will outperform it greatly. I'm thinking of the lower AM2 Athlon 64s beating equivalently-priced Semprons by a fair margin here.
Here are things that are *never* good buys:
1. Parts that are bleeding-edge new. Wait a month or two and prices will drop a lot.
2. The most top-of-the-line unit, such as any Extreme Edition or FX series CPU.
3. The last revision of technology once the newer stuff has been out for several months (i.e. Socket 939 Athlon 64s and Pentium 4s and Ds.) The bleeding-edge prices are gone from the new goods and the new stuff and you're probably left with no upgrade path or a decent part selection on the older goods.
4. Getting a cheap component and then a lot of expensive parts to overclock it. Spending $300 on a water-cooling unit, $100 more on super-fast RAM, $100 more on an extremely OC-friendly motherboard just to clock a bottom-end chip up to the same level that an upper-mid-range chip can reach on regular RAM, a midrange board, and a $50 heatsink is idiotic because there's usually not a $450 difference between an upper-mid-range chip and a bottom-end chip. Plus the more expensive chip will run cooler and quieter and run longer and more stably. I can say the same thing about GPUs too.