I am currently playtesting a spellcaster Eidolon rather than the obvious melee eidolon. Combat in our games usually lasts about an hour out of game but only 6 rounds or so IN game, so in combat I usually get about 5 standard actions to cast spells (the 6th standard action is usually lost to boss mechanics or dealing with situations etc). So I am basing my personal playtest off of combat lasting 6 rounds. I'd really like to see what Ideas the comunity has to play off this same type of "alternate Eidolon build."
Evolutions:
Spell-like ability: is the obvious evolutions used. The problem with it is that the way it reads, it cannot be chosen more than once thus only giving the caster Eidolon up to 3 castings of the same spell.
Breath Weapon: This mimics a damaging spell, with the exception that it requires physical positioning of the Eidolon.
Web: This mimics entangling spells and can be used up to 8 times per day. Very usefull for the build.
Spell-resistance: not a necessity for the build but adds a lot to the strength of the "caster eidolon" and also mimics a defensive spell that is always on and doesn't take up your limited standard actions.
Ability bonus: bonus to Cha so you can cast higher level spells, or bonus to Con to increase your reath weapon and web save DC's.
Limbs (arms): to manipulate a lot of magical things in a dungeon, fingers may be handy. just a guess :)
Feats:
There are a lot of feat support for this in the 3.5 books like "night haunt (CA), the Fey Heritage feat lineup (CM), and a lot of spellike ability boosting feats (like "heighten spell-like ability"). These feats give me more spell-like abilities to add to my line-up for both in and out of combat situations like: charm monster, dimension door, deep slumber, summon nature's ally V, disguise self, confusion, unseen servant, dancing lights, prestidigitaion and much much more from similar feats.
Skills:
use magic device and spellcraft are solid choices. Plus linguistics helps, I know they can already speak all that the summoner can, but a lot of the enchantment spells that I got from some of my feats are language based, so speaking their language will help and you may not want to spend your summoners points on it since you only get 2+ int as skill points. (plus the built in decipher script part of the skill is handy).
Conclusion of the spellcaster Eidolon is that there are more than plenty of standard actions to keep me busy during the 6 rounds of combat that I expect on average (hell I could spend all 6 webbing people if i was half zoned out for the night), thus making the build very viable. I as a summoner can still cast summon spells if I need a more melee oriented pet to play with, lessening the risk of the Eidolons decreased survivability through feat choice.