Simano makes eight levels of mountain bike components. The high end, of course, is for racers and hardcore folk who really need decent performance. The low end is for unsuspecting buyers in department stores or looking for low end shop bikes. These components work poorly, and if ridden enough (5-10 miles daily) will probably break within a year or two. Here's the secret: The cost to make these low end components is very close to the cost of their high end cousins. When an Acera rear derailleur costs $15, does anybody really believe that stamping a similar (but somewhat lighter and somewhat more durable) rear derailleur with XT actually raises the price three fold to $50? of course not. The few advances in lighter weight (forged aluminum vs steel, better springs) and durability (better seals, bushings, and harder pulley knuckles) Would probably cost only an additional $5 or so. But, it the difference in price of Shimano's lowest end component and highest end component was only $15, who would buy the low end stuff? So they jack the price of the good stuff really high, and keep the people who can only afford their lower end stuff stranded with, well, crap.
There is an advantage to Shimano's pricing methods though: the top drawer XTR rear derailleur costs $100, and the second level XT costs $50. Which means that the third level (and still quite decent) third level LX costs only $25. The LX rear derailleur incorporates a lot of "top-drawer" zoot from only a couple of years ago, and yet, costs only 1/4 of the price of their current top of the line mechanism. Sadly, however, Shimano neglects the masses, and while the racing technology does trickle down a little, Shimano prevents thetechnology trickle down to the Acera, Altus, and SIS level. After all, who besides racers would appreciate a component that won't break, is light, and actually works reliably?

The Verdict?

So, for an extra $10, don't buy the Acera rear derailleur, buy the LX. It works better and you'll feel much better about at least one component of your bike. If you are searching for a new bike however, I still recommend a bike with at least an Alivio rear derailleur, as while it is quite heavy( nearly 100g more than the LX and about 25 g more than Acera, I believe), it shifts reliably and will work for more than a year, unlike Shimano's lowest end stuff.

*note* the price difference quoted is from mail order companies. the price difference in shop will probably be quite a bit larger.

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