Influenza vaccines are both safe and effective. They basically use a very depleted flu virus that is not enough to infect a person with influenza, but is enough to stimulate the body's production of antibodies to fight that specific strain of flu. This is the scientific basis for all vaccines, and this one is usually quite effective.
Some people experience mild flu-like symptoms for a day or so from getting vaccinated, such as nausea, light-headedness, dizziness or lack of energy, but many people do not feel this at all. These side effects are also rare, with less than 10,000 reported incidents out of over 70 million vaccinated in 2009.
The vaccine is safe for pregnant women, and for young children, although the injected variety is recommended for patients under the age of 2, while everyone else can take the nasal spray if they so choose.