Party composition: Investigator 2, Ninja iconic, Rogue 1, Bard 1, Medium 1 (me)
There will be spoilers and there won't be spoiler tags. That said, this is a replayable for PFS, so take that for what you will.
Goblins, it turns out, are great targets for a level 1 that focuses on having natural attacks. Unless the die came up as a 1, I was one-shotting virtually everything that walked in this scenario. We came up the back way, which negated the "Tucker's Kobolds" aspect of the area and allowed us to deal with the goblins as unalert packs. The bard and I were the only ones with darkvision, but that made it thoroughly possible for us to see what we were doing. On the average full attack, I was hitting on approximately 2/3 of my swings with average to slightly-above-average rolls. Nobody buffed at all in the course of the dungeon.
Problem encounters
Obviously, the bugbear was painful. We were unable to kill him due to the obnoxious terrain rules for this level and he opened the melee portion with a crit that outright killed the ninja. I spent virtually this entire combat at 1 hp and was blessedly missed repeatedly. I hit on approximately 1/3 of my attacks, typically with a reasonable spread of dice each time.
Construct + cleric was painful. Both the ninja and rogue went unconscious, I ate 3 channels and the construct's explosion during the fight, leaving me at 1 HP as we finished. As is no surprise, the construct's DR was a problem, but not insurmountable.
General play impressions
1. There's simply not a lot to do at level 1. Yes, I was constrained by a lack of spirits by choosing to go 12 CHA, but I can't honestly say that I would have Tranced at any point during the session if I had any other spirits available. As I remarked to the GM, who was leery but accommodating of the playtest material, I could have built virtually the same character via Tengu Barbarian and done more damage, had more HP, and a better chance to hit.
2. The flavor of the Medium is hard to convey in a PFS setting, particularly when players and GM haven't seen the playtest doc, or, in the case of some, even heard that there is a playtest doc. It's even harder when you're playing a dungeon-crawl scenario. I played towards the compulsion, but I was honestly the only person who knew that I had such a compulsion for using the spirit.
3. Attack/damage seemed fine. I consistently contributed to the combats in a meaningful way (read: I killed over 1/2 the enemies because I had multiple attacks). That said, we're talking goblins here. Yes, they have decent AC, but they're not exactly sturdy. Since I was dominating, I took a step back during some combats, particularly if I could justify it due to distance.
Parting thoughts
I'll be playing at level 2 for a scenario, which will get me to level 3 as this was a converted level 1 character that started with 2 xp. Level 3 will likely be the next floor of Emerald Spire.
This class needs to figure out what it's doing. It's a little schizophrenic right now, which ends up meaning that it lacks clarity and direction. As has been repeatedly pointed out, it's the only 4-level caster that isn't also d10 HD and full BAB. It's also tremendously backloaded, so the first 4 levels are very slow and they slightly pick up as you get towards 7, where you finally look like a class that can maybe do things. This leads to a rather boring intro to play that can easily be duplicated by other classes.
Strength is obviously a good choice for spirit if you intend to be combat-focused. Other spirit choices can be helpful, as well, but the first few levels are simply boring. Not qualifying for Power Attack is very painful at level 1 when you intend to melee, but the not-PFS-legal feat in the Medium thread somewhat addresses that pain. Meaningful combat choices typically come from items rather than class features, which is troubling.
I would like to see something additional at level 1 that allows the Medium to contribute more to the party. Maybe that means giving it 6-level casting. Maybe it means an additional general ability or something special for each spirit at level 1. Maybe that something could be that Dual Vessel gets moved to an earlier level. There's just not much going on at level 1 and you feel outdone by anyone who does anything that resembles specialization. At this point, I think a move to 6-level casting is probably the best choice because it will better support non-Strength spirits, give added utility for levels 1-3, and make the play more engaging.