First, positives. It basically works as advertised and allows you to windowshade your windows without haxies and without modifying your system (which is a huge bonus).
There are many different settings, so the app is very polished and complete. The shaded windows play nice with Exposé in 10.6.8 and Spaces (which is vital). You can even move the shaded windows around to other Spaces when showing all Spaces as well.
It seems stable and doesn't appear to slow down my computer at all. It has most all the options you would expect from a Windowshade app including choosing transparent windows of varying opacity instead, etc.
On the downside, you're basically forced to hide all docked windows to the application icon or your dock will get cluttered up pretty quickly because this works by minimizing your windows to the Dock. That's a give and take and I much wish there was some way to windowshade the windows without them having to minimize go to the Dock. This would make it work much more like traditional Windowshade apps I used to see with OS 9 which were fantastic.
Also, there's a slight lag after double-clicking even after choosing all the most optimal settings to speed it up. This is also because of its dependency on minimizing windows I assume. It's not a long delay, but on a powerful computer I get used to near-instant responsiveness in everything I do, so it's a little jarring to see something delay for a half-sec.
On the positive side again, you can at the expense of adding more windows to your Dock and having less space in your Dock, turn off minimizing to app and you'll see all your Windows again instead of being tucked inside your app icons.
Overall, this minimizing dependency is the main reason I'm turned off by WindowMizer but ironically I suspect it's also the very reason it functions so well without having to alter the system.
One glaring mistake is the lack of ability to hide the WindowMizer icon from the Dock taking up previous space. I'll probably end up hacking it to remove it, but it'd be much better if the developer added that as an option in the preferences instead similar to apps like LaunchBar, etc.
I've put it through its paces and it doesn't seem to use much system resources at all. That said, I have run into odd behavior here and there. For example, I've click on shaded windows and they've opened up in another Space. That might have been from moving it to another Space previously or something, but for whatever the reason that's a bug I hope gets squashed.
I haven't yet tested on my Mountain Lion machines, but I'll report back later if it causes trouble there, which I doubt it will because the dev seems very responsive to fixing bugs.
In the end, I'm still deciding if it's worth it considering I have to change my Dock behavior by minimizing windows to apps or don't do it and deal with cluttering up my Dock. I wish I didn't have to make that choice, and hope the dev figures out a stable workaround to that situation.
That said, I'm very happy to see a new, stable, Windowshade app on the OS X scene. I've found Deskovery works better, but I've found it to be buggy and cause conflicts. On the other hand, WindowMizer doesn't seem to conflict with things like DockView which is a critical positive or I couldn't use WindowMizer.
For now, I'm going to continue to use WindowMizer and play with the Dock settings back and forth to see if I can work efficiently with not seeing my minimized windows in the dock. Then I'll try it without minimizing to apps and see how bad it is having WindowMizer adding more stuff to the Dock. I think I'll probably just deal with the latter in the end and be pretty happy with it.