We went to Koh Samui for 12 night in November 2012 and visited Spirit House twice whilst we were there.
We were guided firstly by the excellent reviews on trip advisor that this restaurant already had and were expecting some nice food. The food at Spirit House is absolutely amazing, on the first night my partner had red duck curry and I had a curry with Prawns in (can't remember the exact name) Both dishes were so tasty and the sauces were amazing along with the lovely fragrant jasmine rice.
On the second night we had 2 steak dishes, one in a massaman curry sauce and the other in a lemongrass sauce. Again, the food was so good. On both occassions we had the rainbow leaves to start and these were heaven, the flavours in the leaves were so yum.
The restaurant also has a lovely atmosphere and you get to sit on the floor to eat your food if you wish which made it a unique dining experience. The waitresses were lovely and we were also asked how our food was by (who we presumed was) the restuarant owner.
Although it was a fair way from our hotel it was definitely worth the walk or even a cheap taxi ride up there.
I have absolutely no faults with the restaurant whatsoever and it was personally my favourite of the whole holiday.