I used Swiss cheese as suggested by a previous user, and also added some minced garlic from a jar to the spinach mixture. Still, it wasn't very flavorful and I wouldn't make it again.
The local grocery was out of portobellos, so I had to make do with regular mushrooms. I also used regular canned crabmeat instead of lump - I like the texture better in mixes like this. I enjoyed them quite a bit. As an appetizer, it was AWESOME! I'm serving some to my in-laws tomorrow night (I made extra mix, so I'd have some available). I'm looking forward to trying this with Portabellos, too!
I tried out this recipe on New Years Eve. Four of us were spending a quite evening with good food and wine. We all found the recipe to be bland but all the ingredients we liked. If I make it next time I will try maybe swiss cheese and saute the spinach with some onion and garlic.