Assuming a 17th level summoner (24 cha) and a 17th level conjurer, the conjurer will be able to cast summon monster IX twice per day with a duration of 25 rounds as a full round action. This seems completely reasonable until compared to the summoner, who can cast summon monster IX 10 times per day (assuming a 24 cha) with a 17 minute duration as a standard action. He can also cast it normally if in a bind twice at that level (the same amount as the 17th level conjurer) and he doesn't even have to make that choice until the situation presents it self.
In my opinion the Eidolon is perfectly balanced until you bring into play the amazing amount of battle field strategy the summoner will have at his disposal with it plus the summons. Give it support feats such as Greater Bullrush or Greater Trip and things will begin to drop incredibly quick. I believe that the problem lies in the summon monster SLA, not the Eidolon or the summoner himself at all.
**also with the extended duration, if the summoner expects a large assault he can have an army at his disposal seconds before an attack.
In my opinion the Eidolon is perfectly balanced until you bring into play the amazing amount of battle field strategy the summoner will have at his disposal with it plus the summons. Give it support feats such as Greater Bullrush or Greater Trip and things will begin to drop incredibly quick. I believe that the problem lies in the summon monster SLA, not the Eidolon or the summoner himself at all.
**also with the extended duration, if the summoner expects a large assault he can have an army at his disposal seconds before an attack.