I moved from fenders and modulus to a Thumb fretless. I love the the tone, how ever it is a little neck heavy and ergonomically doesn't feel right to me after playing other basses for so long. How I naturally want to play is not how the Thumb wants to be played.
I have a 5 string fretlles where both PUs are closer to the bridge. I try never to rest my picking thumb on the low B-sting. I'm a purest when it comes to resting my thumb on the PU. That means I'm generally plucking in the same location getting the same tone. In my experience with other basses I have been able to get different sounds comfortably switching between picking locations. On the Thumb I seem like I only have one location. It also seems like I'm always wrestling with the bass. My should feels it when I'm always plucking in the same position. The neck definitely wants to dive on me. It is pretty heavy in general (11 lbs?), but managable.
I personally don't like the on board EQ on the 5 string that much. The PU selector has little effect. The bass and treble effect the tone, but unlike other basses the change is not inspiring. For me the sound is awesome, but I like it for only one color, which setting the Bass EQ flat. I do find myself tweeking my Amp EQ. It sounds good completely flat, but I need to dial it in more with my Amp.
I don't have it setup to slap because it is fretless, but I must admit that when I do slap it, it feels really natural. My thumb and forearm rest in a natural position.
I'm moving towards putting it in the market. Right now it is acting as my main bass, so I need to find another fretless before I move on. I have really grown to like the character of the Thumb, even though I think it is 1 dimensional. When I try other basses in the store, they don't seem to have the same character, so it will be hard to move one.