My first thought is simply this: the game is amazing. Great art, great theme implementation, and great components (my only caveat is that the map pieces have started peeling/splitting on the sharp corners). It's also really fun. This game has been an instant success with my group (ie. co-op enthusiasts).
Second thought: the base game is really well balanced. This specifically true concerning the spirits. Ive played 12 games now and been every base game (plus KS) spirit exept Serpent. About 6 of those games have been vanillia with no Advesary/Scenario, we've played 1 game against each base level Advesary (including France), 1 game with level 1 Prussia, 1 game with level 2 Prussia and Guard the Island's Heart (which seems to sabotage Prussia's level 1 setup bonus), and 1 game against base level France on the thematic map. We have not used the expansion yet. In all of those games except 1 (more about that 1 later), we never once saw an Invader Stage 3 card and often we won within the first two Stage 2 cards. This was with a variety of player counts and spirit team ups. That's pretty consistent imo albiet rather easy (easy for our group; we're good at planning a turn ahead).
The 2 player game we played against France on the thematic board however was a whole another story. That combination is listed as a dificulty 5 (our previous highest was 3) and the spirits in use were Vital Strength (myself) and Shadow Flickers. One thing we didn't notice about the thematic map until the second turn, was that it had 10 spaces per board rather than the vanillia map's 8 which makes it very hard to control and contain the Invaders. This game had us streached to our breaking point by the middle of Stage 2 and we had to make some dificult calls about our sprirt's growth options. Once the Stage 3 cards started playing, we were both doing nothing but reclaiming our cards since those card were the only things keeping us alive. Up to this point we still had a healthy island due to heavy defensive play on my part, but now we realized the Invader deck was about to run out and, even though we had hit Terror Level 3 (thanks Shadows!), we were going to lose before we could kill all 5 remaining Citys. So we just dropped all defense and started a full on fear campaign, and even helped the Invaders blight the island with some of our powers. It was close, but with Vital Strength generating 6-7 fear a turn and Shadows earning around 8, we hit the last turn of the game with 1 fear token left to achieve a TOTAL FEAR VICTORY! One swift phase, Fear generating power later and we won the game! That game took 3 1/2 hours and was miles beyond our other games dificulty wise and that was only half the dificulty possible for the game.
Third thought would be that I'm unsure if the game has a middle ground dificulty wise. My group is (apparently) very good at puzzly co-ops. So I wonder that, since we always beat dificulty 0 games by the middle of Stage 2, if a player a little less action eficient or less adept at planning for the next turn would by extension win a little later, say by the end of Stage 2. Or if that player is not being that efficient in the early game, would they experience a "snowball" effect in which the Invaders win by blight or Spirit elemination by Stage 3? Haven't played with the expansion yet, so maybe it will smooth things out.
My final though would be that it would be a nightmare to try and balance custom spirits for this game. Mainly because of the headache that must be involved in deciding what elements each of a spirit's starting powers get vs what elements their innate power(s) get. So..many..permutations! I love making customs and I've done so for many of my games, but this one looks kind of unapprochable.
Anyway, that's what I think of GtG's Sprirt Island thus far. Rating for our group: 10/10