Yup, what Anklebite said. There's really no reason to ever leave the class and lose eidolon advancement.
Good races: human, gnome, halfling.
Your first 2 feats will probably be Spell Focus (Conj) and Augment Summoning, Cloudy Conjuration would be a good follow-up
Obtain Familiar and Improved Familiar or Dragon familiar
The summoner I'm DMing took Armor proficiency (medium) and battle caster (medium) so he can tank
Toughness (PF version)
Charging feats if you want to ride your eidolon into battle
Quicken Spell, extend spell
Good equipment: A mithril chain shirt, mithril buckler, morningstar, spear, maybe a lance. Metamagic rods are nice because the spell list only goes up to 6th and some spells come early for the summoner, though the spell list got heavily nerfed in the final playtest version. I miss having Haste as a 2nd level spell
Good spells: Enlarge person along with share spell is brutal early on. Shield and Protection from Evil are also good spells to cast on the eidolon with this ability, while grease is a good control spell. Later bull's strength, glitterdust, and resist energy are good, followed by haste and greater magic fang.
Eidolon: I'd say go for the quadruped and get as many primary attacks as you can, if I understand natural attacks in PF correctly. I would start with energy attack (acid) and claws, and add ppunce at 2nd. You want to always be at the maximum number of attacks possible, probably increasing mostly through through a tail and tail sting or tentacles, or through extra limbs (legs) + claws + rend/rake. Later on, add flight, str increase, energy attacks, trip, and large size. For feats I'd generally go with basic stuff like Toughness, Multiattack, and Improved Natural Attack, or maybe go down the ubercharger build.
Good roles to focus on: Tank, buffer, and battlefield control. Between life link and life bond, it's pretty hard to kill either of the two, and the summoner gets all of the good wizard battlefield control and buff spells as well.