Regardless of skill, a brewer produces five units of alcohol for each unit of brewable plant, as well as seeds of that plant. Because of crop stacking, this allows brewers to synergize well with highly talented herbalists and growers (who are more likely to produce larger plant stacks), and can quickly lead to cluttered stills. A brewer will turn "Plump helmet" into "Dwarven wine ", and will turn "Plump Helmet " into "Dwarven Wine ". Both actions fill one empty barrel (despite their different quantities), and presumably take the same amount of time to brew.
Unlike many farming labors, brewing cannot be done without a barrel or pot. This can become an issue if your fortress has more food than barrels, as any spare barrels will be immediately filled with food. Limiting the number of barrels that your food stockpiles can store can help keep some barrels empty for brewing.
Brewing plants always produces seeds, so you won't have to worry about being unable to replant your crop next season.
 Skill, Quality and Thoughts
Dwarves with a preference for a particular type of booze get a happy thought from drinking it. A dwarf whose preference can not be met will never have a happy thought from drinking. Currently booze has no visible quality level, but thoughts from drinking imply that there are factors other than preference that influence the drinking experience (had a fine/pretty decent/wonderful/truly decadent/legendary drink lately). It has also been speculated that drinking home-brewed booze is preferred over imported booze of the same type, influencing the thought.Verify
If there is a hidden quality level, Brewer is a very important skill. If not, Brewers are just glorified Wood Burners. In any case high skill in brewing allows for booze to be made faster.