I wanted to depart from my usual format and say a few quick words about Sram's awesome Hammerschmidt. I don't think this gets enough credit today. In my opinion, it's the most innovate accessory of the last five or ten years. It's expensive, and it has a few drawbacks, but after using one for a year and a half, I will never be able to ride a bike without it.

Pros:

• Clearance, clearance, clearance.
• Instant (and I mean instant) shifting, so you don't have to plan ahead. No more bent gears or thrashing sounds as you struggle up a steep incline in your highest gear. Often, I won't even tough my back gears because I just go back and forth on the Hammerschmidt. Traditional shifting's just too slow!
  
• Easy maintenance

Cons:

• Expensive
• Heavier than a traditional set up (although not by much, and if you're not racing, who cares?)
• Slight drag on the overdrive (higher gear). This isn't a problem for me because I only use the higher gear going down, so a little drag won't kill me.
  

Obviously, this isn't for everyone. But it's for most. If you race xcountry, don't buy it, but if you don't race and like to ride all over the mountain (enduro, trail biking, all-mountain, etc) there isn't a better set up. If you're on the edge, just get it--I promise you won't regret it. I think that too often we focus on weight and efficiency when we aren't in a racing setting. Who cares if my bike's a couple pounds heavier? I'm not in a race, and I still make it up all the climbs. And I have a lot more fun going down and don't need to worry about catching my crankset on a log or bending a rim. Just buy it.