So, I've put together my level 1 Medium and I'm going to be playing this Saturday in PFS, running Emerald Spire 1. The character is a rebuild from an unused character with 2 experience, so I had reasonable starting gold to work with. I'm not saying it's ZOMGTHEBESTTHINGEVER, but the character fulfills a specific niche concept I'd been sitting on for years.
At first blush, I think the BAB/HD should be full/10 instead of mid/8. Beyond that, though, I'm failing to see how this class has fewer options 1-8 than a Fighter. The only thing I see that is particularly painful is the inability to take Power Attack until level 3 because of the mid-BAB. Beyond that, it seems as though you can shift around to have a few different relevant options pretty quickly with the use of feats. If you intend to use certain spirits consistently that grant feats (see: The Beating), you can use them as the prereqs for other feats (Imp Grapple). That means that, at level 7, when you have The Bear as primary and The Beating as secondary, you can turn into a huge creature and grapple pretty effectively.
There is depth here. I agree with the BAB/HD argument, but I don't see the options as significantly limiting. They're not nearly as power creep as other options I've seen out there, but it just needs a bit more exploration.