Bittering Units - Measuring Hoppiness

So in my reviews, I used stats that included the enigmatic "BUs" (sometimes rendered "IBUs") . This stands for bittering units (or international bittering units), and it's a standard chemical equation for how much alpha acid from hops is utilized in a beer. It's fairly useful shorthand for estimating how bitter a beer is going to be, with a few caveats. I'll try to use them as often as possible when writing about beers, so this handy primer should be valuable.

What Hops Do
Without hops, beer would varying degrees of sweet (depending on the amount and fermentation of the malt), something like a breakfast cereal. Hops, which have a complex chemical structure, are used principally for bittering--they offset the sickly sweet of malt and make a beer drinkable. The chemical in the hop that creates bitterness is known as "alpha acid." Hops have a multitude of other chemicals, though (some of which are not well understood), and these contribute other flavors and aroma. Each hop has its own character, so those aromas and flavors vary. So, two beers hopped at the same alpha level but with different hops will be roughly as "bitter," but will taste different.

Caveats

1. Bitterness is a perceived, not absolute, quality. There are some hops I find intensely bitter like English Target. To my tongue, they produce a far more bitter beer than one made from an equivalent amount of alpha acids derived from, say, Chinook.
2. Bitterness Units are only a mathematical calculation of the percent of alpha acids per ounce. The more malt used in a beer, the less bitter it will be. So if a pale ale has 35 BUs, it will taste pretty hoppy, but an imperial stout with only 35 BUs would be overly sweet.
3. Much of the flavor that comes from hops isn't bitter. BridgePort IPA has 50 BUs, but few people would describe it as "bitter" because the hop concoction (there are five varieties) is so citrusy.
   

Using BUs
An average beer is around 5% alcohol by volume (1.050 OG). Roughly speaking, this is how bitterness would be perceived in that beer based on BUs:

BUs
10-15 - almost no perceived bitterness (industrial lagers)
14-20 - mild (golden ales, wheat beers)
20-30 - some bitterness (brown ales, English bitter)
30-40 - pronounced bitterness (pilsners, pale ales, porters)
40-50 - sharp bitterness (IPAs)
50+ saturated, intense bitterness

Some beers are super hopped, in part because of high malt bills. A few examples:

Strong ales - 40-65 BUs
Imperial stouts - 50-80 BUs
Barleywines - 60-100 BUs
Imperial or double IPAs - 65-100 BUs