As a Austin resident, ACL festival has been a huge success. It is a 3 day festival that goes on in the largest city park, Zilker park. There is about 5-6 different stages all across the park, general admission, and it is a long 3 day music festival. All the info is at
www.aclfestival.com
If you really want to go to ACL here are some good tips.
1. Buy your 3 day ticket passes now. They will sell out very soon ($90 tier 1, $100 tier2 passes already sold out)
2. They plan this when UTexas has a non football game this week. All the hotels will fill up fast for the festival.
3. It is a great festival, mainly because Austin is a great city to visit. You will see hundreds of acts
4. Be advised, just because its september, it will still be freaking hot. This is Texas. Past 2 years, it was clear sky, mid 90's all day, dusty and just severaly hot. So you need realize you'll see a lot of bands, but it is crowded and is hot all day long.
Its a long festival, but you can see a lot of bands, and it seems to get bigger/better each year.