My experience with the Summoner was short and sweet. Our group added my play test character Jevis. He appeared in a large dungeon encounter. As a 9th level character he was very well prepared for the big battle, so I thought. I was attacked by a Thune Hulk and died in one round. The Thune Hulk closed in faster than I expected. I was unable to run and my Eidolon kyynel watched in horror, as I was wiped out. I think if HP were shared both ways he would have had the 12 HP he needed to survive the round and use my transposition ability. I enjoyed the experience and would play the class again. We also wondered if my Eidolon kyynel would stay to avenge his friend or go back to his home plane at the end of the round I died.
Consider allowing hit points to be shared from Eidolon to the PC. Also clarify if an Eidolon returns to is home plane if the Summoner dies.