When I went to BGW in March with my Dad I told him we wouldn't need BG's Quick Queues. He didn't listen to me, and bought some anyway. We only used them twice, but it was completely unnecessary. But it was little crowds so all the rides were basically walk-ons.
For the QQ's, (correct me if I'm wrong) I think it's like an extra $20. You get a little card that shows all the rides you can use it with. I believe you can use it for all coasters except Verbolten, and some other rides like Darkastle, Mach Tower, and I think Europe in the Air. You can use it once per ride, but you also can use a your "bonus" ride, which lets you use it on a ride that you already used the QQ on (and you can use your bonus whenever you want).
The two rides we used it on was Darkastle and Alpengeist. All it did was just skip the first half of the lines and just lets you out at the loading stations, where you can choose where to sit (I'm assuming it's like this with the other rides). Also if the wait time for a ride is fifteen minutes or less, you can't use it (I'm not sure if it's like that all the time but at least that's what happened when I was there).
While I've only been to BGW once and not on a crowded day, I would say you're better off without the Quick Queues.