I've played on in a Kingmaker PBP for a few levels now (over the course of several months, PBP play being a bit slower than average).
The Eidolon, being an NPC, only gets 1/2 it's HD worth of hit points. At first level, he'll have four hit points, plus whatever he gets for his Con bonus. If you've played a Druid before, you'll probably be a bit surprised at this squishiness. Take Toughness for it's bonus Feat, if you want to use it (and, unlike a Druid companion, animal barding to boost it's AC is not an option).
Life Link is garbage at lower levels. The Eidolon doesn't die until -10 to -13 hit points, and you can't use the power until it's about to die. At first level, you might only have ten hit points, and if the Eidolon is at -12, you can spend all 10 of your hit points to save booboo from death and leave him stabilized at -2. It's *far* more effective to just let him die and stick to SLAs for the rest of the day.
In fact, at lower levels, the SLAs are pretty freaking amazing, particularly in combination with Spell Focus (conjuration) and Augment Summoning (which you can have at 1st level, if human).
I found grease and mage armor to be useful spells at 1st level. Grease is almost always useful at low levels, and mage armor is occasionally useful to buff up the Eidolon's weak defense.
Since, between 1st and 3rd levels, the Eidolon has died within the first rounds of almost every combat, it's very useful to have monster cards or quickie stat blocks handy for whatever flavor of (possibly augmented) eagles and dogs and whatnot you'll be summoning with the SLA. Slowing down the game while you figure out the stats for a celestial augmented fire beetle will earn you no friends.
The Summoner himself, once he's gotten his grease spells and daze cantrips out of the way, is not a terrible combatant, and can provide some crossbow bolts or even reach attacks with a longspear.