We have an Archivist/Alienist in our group: a player who is very fond of Call Of Cthulhu seems to have rolled up the character to sort of subvert the whole D&D game, since he's not as big a fan of D20 as the rest of them. ;-) He modelled him a bit on Giles from Buffy, gentlemanly and intellectual if a bit strange.
It is indeed a good combination, the Int/Wis split of the Archivist allowing the Wisdom losses from Alienist to not sting so much. Rather than summon just Pseudonatural critters, though, I gave him access to monsters from the D20 Call Of Cthulhu book: so sometime he brings forth a Star Vampire or a swarm of Mi-Go, rather than just "a blobby shark thing" or whatever. I find that's a lot more flavourful, and because he's already up on Cthulhu mythos it enables him to come across as appropriately "Far Realms expert"-ly. Plus, as others noted earlier, you otherwise lose access to more potent high level summons, which might sting if the game goes on for any length of time.
He also uses his Dark Knowledge (combined with ace skill points, partly due to a very high) to serve as the party's generic "what the hell is that?" guy. This allows him some small, bard-like buffing power, whilst also having a handy tool for me as the DM when players could use a hint. Combine all that with his broad spell selection (being theoretically able to pull such strange spells as Reincarnation or Lightning Bolt into his collection, from Druid and Adept respectively) and he's a funky char. His biggest flaw is HP, since his Con is awful and Alienist don't give out that many HP.
As for feat choices: my party don;t use metamagic feats all that much, although I think the player was angling towards a Sudden spell or two for the future. He's not necesarilly an "optimizing" player, so his choices here wouldn't help you that much anyway. ;-) But certainly, his main contribution to each combat is to summon critters to fight, using his spell slots to buff them: focus on getting those as useful as possible, and when stuck, Skill Focus to your most useful Dark Knowledge skill will help. (The feats that let you expand Dark Knowledge's uses to other races may be worth checking out depending on campaign)