Late to the party, I know, but was just looking for opinions on the different Spitfire pianos online and ended up here, so I thought I might as well share my opinion
I have Cinematics Soft Piano, Felt Piano, Mrs. Mills, Firewood Piano, and Intimate Grand Piano from Spitfire, and there's something to be found in each one of them. I won't use any of them if I want to "play piano" or I really need a dynamic or realistic performance (for that I've got the amazing and yet still affordable Hammersmith Pro), but all of them are beautiful in their own way and find their way into my music. You just shouldn't expect them to be solo instruments for a classical piece or anything, they are there to do their job: sit right there in a mix with other orchestral/cinematic instruments.
Call me crazy (I know this is going to sound silly) but Firewood piano is probably my favorite, just because it's such a wild and unpredictable beast (dynamically), that it
will affect your music, and I mean it in a positive, creative way. Suddenly you hit a note slightly harder than the rest in a passage, and bum! There's this beautiful grace note. It has some guts and it's a very "intense and temperamental" piano.
I also love the felt piano, it really sits pretty well into most cinematic/symphonic pieces.
And of course the Intimate Grand is just SO beautiful when you need to go for that soft, dramatic sound.
When I bought my first Spitfire piano (that was the Felt Piano I think) I was disappointed at first, waiting for... you know, the feel or dynamics of a real piano, but they grow on you and are great resources for your instrument repertoire. All of them have their strenghts and can add a lot to your music IMHO, and they are really good value for the money.