This subject has been covered many times by many people, so after Carl asked, I thought I would put my perspective on it. Again, this is what works for me, not necessarily what is the best thing to do.
I said before the race "I need good crew more than I need a pacer".
I do fine running alone hours on end. In fact, I remember so many details of the race because I tell the story of the race while it is happening. I find this helps with my concentration, and avoids the stupid mistakes (missed turns).
I am still not 100% sure what a pacer is supposed to do. Some say it takes their mind off the pain they are in, some say it helps them push on, some say it helps them stay focused, some use pacers to help them stay on course.
For me, I see a pacer as someone who keeps you focused and maybe pushes you. Therein lies the problem. You can't always push in a 100 mile race.
Around miles 20-25, I remember saying to the group I was with "I am backing off a little, you guys are smokin'" It wasn't that they were running too fast a pace for me, it was just at that moment I was exerting more than I wanted to at that point in a race of that distance. I ended up catching up to them, and finishing ahead of them. I listened to my body.
I fear a pacer would pound the crap out of me, and make me run harder at times when I should not.
I remember Kevin G commenting about a pacer (maybe his). "Enjoy it, he does not let you walk... at all." I think he was saying that tongue in cheek, but if it is close to accurate, it could destroy the non elite runner.
I knew where I needed to back off in the 80-95 mile stretch, and I knew where I needed to go for it. A pacer would have been good company, but would have not changed how hard or when I ran. The only place they would have been able to lower the boom is in the last 4 miles. They would have had to been SCREAMING at me, though. And... talking some serious smack. Probably would have had to bring my mother into it.
I did end up passing 4-5 people from mile 62 who had pacers. I don't think that means anything. It could mean I don't benefit from a pace IN THAT RACE. Superior... different question. If I was still in a funk at mile 62, a pacer might have helped a lot, so who knows.
So, in conclusion I would say this.
1 - Figure out what you want a pacer for.
2 - The pacer should know the course well.
3 - The pacer should know your performance level, and intuitively know what is left in you.
4 - Don't let the pacer sacrifice crew responsibilities.
I did enjoy having my crew pace me at the final lap of Zumbro 100k this year. I was just sucking wind for the last 10, so it took my mind off it. My 1.5 mile burst at 57 came from being passed, a bigger motivation than anything.
So that's me 2 cents.