Building on what others have said, I'm attaching a "Shimano Best to Worst" list.

It's worth reading this thread: http://www.bikeforums.net/showthread.php?t=165031

It started as a question about why some freehubs/freewheels are quiet and others loud. What it evolved into was a discussion about various levels and brands of component with some very specific information about differences between them.

Repeating something I said there, I work at a bike co-operative that has drawers full of components. In the case of free-hubs, we have generic Taiwanese stuff right up to Deore LX or XT level. Acera, externally, looks very similar to XT.

As one fellow pointed out in the above linked thread, he has an Acera hub that's lasted 60,000km with basic maintenance.

Perhaps the higher level gear is lighter weight or has other technical gee-whizery going on but if you don't measure your bike to the nearest gram, don't know or care about the gee-whizery (like the replacable spring in an XTR derailleur versus XT), why invest that kind of money? Investing in regular service would make more sense.

I know little about SRAM but in Shimano, there seems to be a sweet spot where you get high value for your dollar. Any lower quality level becomes frustrating and at higher levels the law of diminishing returns applies.

I'd avoid current Shimano MTB components below perhaps Acera level and Deore level is quite nice.
In road gear, Sora is fine.

Under the heading "they don't make stuff like they used to," older Altus components (c.1993 perhaps) are actually quite good, much closer to current level STX. Here's a link with Shimanos heirarchy over the years: http://home.ca.inter.net/~kroberge/hierachy.html

Hope this helps.
CJ