My party really seems to have taken to the Summoner class, and it came as little surprise that one of my players wanted to join the playtest. Neiji, gnome drug-dealer and general causer of mischief converted from an air elemental blooded sorceror to a Summoner with the quadruped (essentially wolf-like) eidolon.
We've run through the first mod, and just finished the play portion of the second. The party consists of the gnome summoner, a Human Wizard, a half elven paladin of Iomedae, a human paladin of Asmodeus (I allow it, dagnabbit) and a gnome druid (with polar bear cub)
I'll list out the things that I've noticed so far;
Summoner is very flavorful and doesn't step on the feet of the parties Wizard at all.
Summon Spells, at these low levels (2-5), have NOT slowed gameplay. The player has been good about having stat cards for all of its summons
Eidolon and summons have in no way mitigated the usefulness of the rest of the party.
I want to say that the Summoner has too many hp, but then I'm drawn to compare it to the parties druid, and the two seem to be pretty much in line with each other... the Druid's pet is only slightly weaker then the summoners, but the druid itself is stronger so I think it's a wash.
I don't like falling back on Protection from Evil, and I can see that I'll have to as the campaign progresses. Augmented Summons are capable of doing some crazy stuff that I won't always have a decent answer for, but that's no different from anything a conjurer or druid can do so no real foul there.
I'm not sure about summon monster. I almost think the class should have a different summon list or something to set it appart from the conjurer a little more? I don't think it's overpowered (you only get one action a turn either way, and the full round to cast has hurt the player a time or two).
So;
Low Levels 2-5
Players are all enjoying the Summoner so far
No noticeable, negative impact on game balance, play, or flow.
I run encounters 1 to 2 CR higher then normal on average. I like it that way.