With so many new fruit IPAs out this summer, it's surprising there isn't a shortage of grapefruits. Or maybe it isn't, since most of these beers don't use whole fruit or even have any fruit sugars in them. Fruit peel, zest, or extracts are most commonly used, and create a fruity aroma reminiscent of those exotically-flavored seltzers, but do little for the body and taste of the beer. These fruit beers are only slightly more complex than they were before the fruit flavor was incorporated. If the flesh or juice is present during fermentation, the fruit becomes more deeply integrated into the overall beer. If juice is simply added to the finished beer, you get a shandy.
So it looks like there are 3 subcategories of fruit beer currently popular: 1) beer with a flavoring element only, such as citrus peel (most fruit IPAs are here), 2) beer with some fermented fruit sugars (like Belgian krieks), and 3) beer with only unfermented fruit sugars (like the radler or shandy). Established beer styles from old traditions fall under each of these subcategories; fruit in beer is in no way new. There is also not one best way of using fruit that should be employed in all fruit beer. A brief exploration of some classic fruited styles will show this.
In the first category are Belgian white ales or witbiers, which traditionally boil orange peel and coriander in the beer for a brief amount of time. These additions match up well with the natural lemony, pear-like fruitiness and peppery spice created by the Belgian yeast. Hops are used lightly to let this synergy shine without distraction. The witbier's magic comes from its soft fruit flavors that quickly recede to a refreshing, spicy, dry finish. If fruit sugars were present in the beer, it would be too juicy and sweet, defeating the purpose it serves so well by just using fruit peel. Some brewers have used fruit juice or puree in the witbier style with success, but these beers aren't really Belgian-style witbiers. Their body, acidity, and dryness are very different than traditional white ales, and brewers would probably entered them as American wheat ales at a competition for a more fair evaluation.
On a side note, witbier and its fruitless, Bavarian cousin, hefeweizen are often served with a slice of fresh citrus. Most brewers decry this practice because it kills the beer's head and muddies the specific fruity flavors they worked so hard to produce in their beer.
Fruited Lambic beer is perhaps the oldest example of the second category. In the Senne Valley of Belgium, the practice of letting beer spontaneously ferment with wild microbes and aging it in oak barrels produces what is called a Lambic beer. Often large amounts of whole fruit are added to the barrels, sparking another round of fermentation which consumes nearly all of the sugars from the fruit. These beers, and the successful foreign interpretations, are the most complex of all fruit beers. Many strains of yeast and bacteria are given months or years to convert malt and fruit sugars into a wide range of acids, esters, and phenols. After all this magic, the barrels' contents are blended to produce the most desirable and consistent product. It's like wine-making, but with more ingredients. These beers are extremely dry and often mouthwateringly acidic, and by allowing the fruit to be fully fermented, that character is not sacrificed. Lambic beer is all about the complex flavors found in extensive fermentation, so a different method of using fruit might be distracting. Versions with cherries, called Krieks, have a unique spice almost like cinnamon to them, along with almond and sour cherry flavors that would not be present without fermenting the cherry sugars.
Some simpler sour beers, like the Berliner weisse, have fruit and its sugars added to the finished product, but these beers are not all about the glory of fermentation like Lambics. They are quickly soured, creating a lemony tartness from lactic acid that is often tempered with fruit. There are many fruited Berliner weisses and goses out there that are perfect for some tart, fruity refreshment, but they are not, like the attention-grabbing Lambic, perfect for reflections on complex fermentation. An even more accessible fruit beer is the shandy or radler, which anyone can create by mixing any beer with fruit juice or soda. A little lemonade and some fresh herbs (like basil) can make a cheap beer so much more appealing.
Many people still see fruit beer as a very unserious drink and, unfortunately, sexism abounds in the subject (fruit IPAs are still masculine enough for men, though). The truth is, fruit has been used in beer which has been drunk by everyone for many millenia. Some of it has been sweet, some strong, some sour and challenging to the palette. You'll find that it's enjoyable in many different applications and styles beyond that blueberry light lager, if you give it a try.