I went for a day in 2004 and the whole weekend in 2006, haven't been back since though.
It's a big festival, the capacity is around 80,000 these days. Lineup is pretty varied - it's definitely a 'rock' festival rather than a 'metal' festival. Metal usually plays a big part though. Donington Park is a legendary arena for rock, so in that respect it's worth visiting at least once.
In all honesty though, I didn't find the atmosphere to be that great. Way too many idiots who think its a good idea to set (other people's) things on fire in the campsite, and generally a lack of mutual respect that needs to exist at rock festivals.
If you're looking for a metal festival in the UK, I recommend Bloodstock Open Air. A much better atmosphere, and (in my opinion) much better bands than Download.