I got my Sinden this week so I can give a detailed and accurate opinion. The first thing I noticed is the trigger on the Sinden is quiet and smooth as opposed to the Aimtrak which is clicky. Next, the Sinden has a pump action, 4 buttons and a d pad to the Aimtraks two buttons. The pump action works as a reload and is so much better than having to shoot offscreen.
Build quality is solid, it looks great, and the cord has ample length. With the pump action and sight notch at the top it didn't fit in my Aimtrak holster so you will need to order one as an extra perk or find an aftermarket that specifically lists the Sinden as compatible.
Aside from being able to lose the sensor bar, the Sinden destroys the Aimtrak in terms of accuracy. It doesn't matter if you change your body's position and you don't have to recalibrate. You point and shoot. Because the Sinden is detected as a mouse you can make speed adjustments via the mouse settings in the OS.
The software has a lot of settings to play with and is necessary to make the lightgun work. For an arcade cabinet I set the applications exe to load on startup (run -> shell:startup) along with my frontend so I don't have to find the exe and click on it every time I want to play a lightgun game. By default the Sinden's white border is bigger than I preferred but you can adjust both the border size and the color of the border. Andy includes documentation but you don't need to use all the advanced features to make it work. He goes over the basic first steps but once that's done you're good to go. One thing that will help a lot is the software includes presets on the configuration page such as "thin border" that you can select and go to the different configurations pages within the app and see what the values are to get an idea of what everything does and tweak accordingly.
When the software is running, pressing the back right button will turn the border on fullscreen, pressing it again will change the border to 4:3, and once more to turn it off so you won't have the border showing when you play non lightgun games or in the game menus in the frontend.
Other than this, the usual MAME mapping is required. Setup your Lightgun X and Y mapping, the trigger button, or anything else you normally do.
In my opinion the Sinden is everything it claimed to be and more. There is no substitute.