As with many programs I've used, there are a few things which need to be fixed or that I think should be changed in Mosaico, but overall I find the software very useful and I've had a stable experience with it thus far. I saw people mention frequent crashes, but I guess either I'm just lucky here or it's been fixed by now in that regard, as I haven't had it crash yet. The rest of this review is mainly reports of minor bugs I've encountered and new feature suggestions for the developers.
I mainly am interested in the manual tiling of windows with Mosaico, and that works just fine for me. I added several different optional positions to the manual arrangement screen through the options to allow vertically dividing it into thirds. It's great that they can be added or removed for larger or smaller screens to minimize clutter.
I have a few suggestions about the manual tiling feature. I use a touch-screen monitor, and while it is possible to touch and drag windows easily enough to position them with Mosaico, I believe it could use some additional options for better taking advantage of multi-touch displays. I suggest the ability to tap outside of a window with a second finger while dragging the window to cancel Mosaico's tiling (which I prefer to leave enabled at all times). Reaching for the mouse and right-clicking seems pointless and isn't easy to do when touching the screen already, and hitting Q on the keyboard also requires another hand. I'd also like it if I could leave manual tiling disabled most of the time and drag with three fingers anywhere on a window to engage tiling (similar to Ubuntu's three-finger drag for window positioning). Besides these things, while this isn't important at all, I thought it would be good to allow the user to change the colors of manual tiling from the default RGB to custom ones, to make this feature seem more like an integrated/native part of the operating system. That's just a "polishing" thing which could come later in Mosaico's development, though.
I briefly tried out the auto-arrange feature, and while it produced awkward results each time for me, I believe it could potentially be very useful if it sees some improvement. I also think there should be a keyboard shortcut for auto-arrange for when the user doesn't have Mosaico's main window open. When I used auto-arrange, it made one window I had on-screen not show up at all, another tall and narrow, and another small; this didn't cause the screen to be even close to completely filled by the windows. (The test windows were Opera 27.0, MediaMonkey 4.1.5, and Windows' File Explorer.)
Presently, it seems like Mosaico can't handle alternative taskbar positions very well when restoring saved layouts. I have mine on the left side of the screen, and Mosaico seems to ignore that and have my on-screen windows offset beneath the taskbar, leaving some empty space on the right side of the screen when I restore a layout.
Besides these things, Mosaico should also be able to handle arranging skinned windows, namely Steam's main one.
The main system on which I've used Mosaico uses a 27" touch monitor with a native resolution of 2560x1440 pixels and Windows 10 Tech Preview.