The summoner as it was released the first time was an excellent class. He had good spells for summoning, good class abilities, and a good companion.
The problem with the class was some combination that made it overpowered. Taking evolutions and using abilities normally would not make it so, only looking for extreme power one would find.
The problems were:
(1)Too much power for the eidolon: The eidolon could surpass a fighting class when using armor and natural armor evolutions together, the eidolon could make unlimited natural attacks, the eidolon could have immense amounts of strength by taking size evolutions.
The problem with armor gave birth to the nonsensical "armor interfere with the eidolon' link with the summoner". There must be a way to solve this better. Like: "Improved Natural Armor(ex): The Eidolon's hide grows thick fur and rigid scales giving it a +2 bonus on natural armor. The plates and scales make wearing manufactured armor impossible for the eidolon." This way the problem would be solved as well.
The unlimited natural attacks problem were perfectly solved.
The problem with overpowered giant eidolons continues. The size increases are still a must do if you want a strong eidolon. Charisma, Intelligence and wisdom based eidolons are still not viable. Medium eidolons are not that good either.
The Hd reduction was IMO the deepest cut in the class. If you wanted a balanced, weapon wielding medium eidolon, you are going to find something frustrating. Also, HD-based evolutions are now useless. Example: the frightful presence evolution: the eidolon will not afect most of the enemies he'll find, maybe just weak minions. At 20th lvl, it affects only 15hd or less creatures. The eidolon became a fighter as good as the summoner, ending with BaB 15(!).
(2)Spells at early levels:The summoner had access to certain spells at earlier (spell)level that other casters.
This was not a problem at all. the summoner has a slower spell progression and he would not gain those spells earlier than anyone. If the problem was with magic item creation, a footnote would have solved perfectly saying that the spells should be considered of their original levels for magic item creation.
Now the summoner can't cast spells like summon monster IX. Some one will say: "but he has the SLA!" yes, but he can use just one a time. So the cleric and the wizard can just summon new creatures round after round while the summoner keeps just one. And better, he learns a lot later than everyone the summons spells that give name to the class(!)
(3) the SLA could create a summon spam:
I could never see this as a problem. Any spellcasting class can do it. And now, they can do it better than the summoner.
I loved the idea of the class, and I think it can still be a lot better. The summoner and the eidolon still need to have other thing mechanically than combat. I find doing a social or mental eidolon impossible with the rules as they are, and hard to find a social spot for the summoner.