My MacBook Air's battery would last about 30 minutes before dying. This was because Lithium-Ion batteries can only be recharged so many times before they start loosing charge.
The repair went great! I bought the iFixit battery and essential toolkit. The repair was very easy (just make sure you have the right tools for the job).
Buy the iFixit battery, it's worth it. The battery that I got holds 7500 mAh. That's 500 more than the battery that came with the computer. When installing the battery, insert the connector BEFORE you screw in the battery. Otherwise, you end up trying to mess with the connector with a pair of tweezers