Great to see the interest growing. I'll just jump in off the top of my head...I'll think of more as I get more acquainted with Mari.
I've been using Mari for almost 2 weeks now. Initial thoughts are that Mari is a useful tool for me. I am slowly getting used to the projection painting. I never gave Zbrush Projection painting enough of a chance.
Everything is really snappy and responsive for me. The viewport interaction has been great. I'm having a cursor issue where it often vanishes as I move from viewport to menu items, but appears again after clicking or moving back to the main viewport. Very annoying. Support has indicated knowledge of the issue.
First thought is that I feel limited without being able to paint in a 3d environment. Not being able to spin the model around and paint directly on a model feels very restrictive.
Mari has a really nice set of Projection Masking techniques that get better and better as I get more comfortable with them. The Edge Masking is a nice Incidence Gradient that is very easy to understand, especially if you know how a gradient works. Being able to combine the masks is really useful.
The Brush Engine is great. Lots of control and variety. Custom Brushes from Photoshop is smooth.
I really miss not having a Zbrush "Drag Dot" brush. Please code one soon!
Not being able to view images in the import dialog is surprising. They show up in the shelf....why not the import dialog ?
Some of the interface space is poorly used....lots of dead pixels cutting into the viewport area.
Now for the big problem I'm seeing. The example will be the Model from the CG Challenge. The Model has 50 UV patches. So, a minimum of 3 maps per UV patch, Diff,Spec and Bump..equals 150 individual images ( If I'm getting this wrong, please let me know). I started to port some of those images into modo just to see how they were looking. They looked pretty good. But...I'm having to do each one manually...gonna take forever and a day if I want to use modo for a beauty render. Even the tutorials over at Digital Tutors made the process look tedious and mind numbing.
I can see an opportunity for a "GoMari/GoModo" function.
Modo and Mari should eventually play nicely with each other.
For small scale work it wouldn't be to bad, but damn....try putting 150 images into modo for a render...if you have a smooth way to do it, please share. I'm a self trained CG artist and some things escape me, this could be one of them.
Again, just an initial response to Mari. I look forward to learning more.