We have been twice in the last couple of years - the first time in the school holidays (QBot) and the second time just before (no QBot). I would say if you're not going during the holidays - and with pre-schoolers you might even be able to find a midweek day to go - then it's definitely not worth the cost. But if you're tied to a really busy time, then it's well worth it.
That said though, with a 3 year old, you won't necessarily be rushing towards some of the more adventurous (and therefore popular) rides, so even if you were to go at a busy time of the year or week, you might well be able to avoid buying one. They are really expensive, especially after you've got in at the gate using as many cereal vouchers as possible!
Last time we went (when we didn't buy one) we got there as it opened and I went to ask the guy at the QBot desk whether he really thought it was worth it on that particular day. He and a couple of colleagues consulted with each other, looked somehow like they'd been rumbled and then said no, probably not! If you don't get one, make a bee-line first thing for what your boys really want to go on. Queues obviously get longer as the morning presses on..
Have fun! I do think it's brilliant there. Your boys will love it (and you too, hopefully!)