Summoner itself isn't that bad. Though the Eidolon makes it similar to playing two characters at the same time, it's still balanced in several ways: the Eidolon takes 10 minutes to summon, and if ANY status effect hits the summoner - such as Sleep, then it's "poof" goes the Eidolon. It ends up having less Hit Die than the party's actual fighters, making it akin to a class-bound Cohort (like you'd get through the Leadership feat). The spells on the Summoner's class list are sub-par, and the Summoner's 'summon monster' spell-like CAN'T BE USED when the Eidolon's out! So it's a choice between a single powerful monster and a handful of significantly weaker monsters.
Two Archetypes for Summoner sign trouble, though: the Synergist. Rather than summon the Eidolon, they become it. In a point buy game, this means that the character can power-up it's mental scores and leave the physical scores for the Eidolon-form, making them considerably more powerful. The other is Celestial Commander. Celestial Commander removes the Eidolon entirely to boost the Summon Monster spells, and give them abilities... they shouldn't have. Similar to what a Cleric should have, really.
The others, like First Worlder (which turns your Eidolon from an Outsider to a Fey), are quite flavorful and add a lot of potential RP to it. Far more than Synergist and Celestial Commander do, at least. Looks can be deceiving - a bard with 'summon monster' and a conjuration-specialist wizard surpass the Summoner in flexibility, while the Summoner does one thing really, really well... it's still a one-trick pony.