ATTQOTD: I used a pace group for 18-20 miles during Rocket City Marathon Last Year. This group was small, maybe 8 people after the first couple of miles. It got me a 38 minute PR and sub-4 for the first time.
I've also latched on to a pace group when I was having a bad day. And one time I let a pace group (of 1 at this point) 'goose' me to a PR. The pacer told me how much of a cushion he had so I knew I needed to beat him by about 30 seconds to get a PR. I'd slow down, he'd catch up, I'd step on the gas (Prius-style
@camaker), I'd pull ahead, and we'd repeat it.
Pros: Leave the pacing to them, allowing you to concentrate on something else (or nothing), strength in numbers, pass time by chatting with other members of the group, wind block on windy days.
Cons: It can get crowded at water stops. You may have an annoying person in the group.
I would definitely use a pace group again, especially when going for a PR and/or a magic number. Most of my pace group experience has been in smaller races, not at Disney. I would be hesitant to use a pace group at a large event due to crowding.