I think it depends on what you want to learn. Most of those I know who learned on their own don't know how to read notes. Kung jamming lang naman, ok na yon. But eventually, note reading can bring to a higher level of guitar playing. The guitar schools teach technique and note reading.

You can learn the same thing from friends or by watching the pros play, but sometimes friends don't teach very well.

Taking lessons also helps you remain committed to learning. Sometimes kasi, people just loose interest when they find the progress to be slow.