I have played an archivist from level 3 through 7, so I have a decent opinion of them.
First of all, the are THE class to play if you want to play Mr. Knowitall. The also make a decent support class, similar to a bard but because it requires move actions you still get to do stuff. However, they are better support for melee types, they are weaker if you have a lot of casters in the group.
As far as spells go, they look better on paper than they are. You have access to a huge spell list, cleric, druid, any cleric domains, etc. You can pull in some very unique spells. However, a large amount of cleric and druid spells are designed for the fact that you have good hp and armor, and are fighting in melee. Archivists with their armor proficiencies and low hitpoints don't fit this as well, as many of the common cleric and druid spells just don't work well for you. And of course, it gets very expensive to pick up those unique spells, as you have to get a scroll to use it.
Overall, I think its a good class, but its a very niche one. If you want to play that niche, then definately give it a try. But I don't think its a powerhouse, having seen an 8th level dread necromancer in action, THAT is the powerhouse of the heroes of horror book.