1) From Brussels, you have to take a train to Leuven (20 min more or less). The train ticket is free with your festival ticket. You'll have to print it before you leave. At Leuven station, there are some free shuttles that will bring you to the Festival site. You can bring your tent of course. Can you take it on your flight, I have no idea how it works, you'll have to check with your airline company.
2) The weather can be either pretty hot or rainy. It depends of the years. It's usually quite good but, never forget it can rain. The atmosphere in the camping is great (The Hive). The camping is at 15/20 walking distance from the festival area and it's really great. Charging phones is free. You have to pay for showers but I can't remember how much it is.
3) It's like 3€ for a beer. Tickets are at 1€50 and it's two tickets for a beer. It can go to 3,4 or 5 tickets for food wich is totally fine.
4) It's much more about music but the atmosphere is really chill, people friendly so you'll have a great time.
So yes, it's worth it going there