I'll start by saying that this controller is a must-have for Gear VR owners. If you were on the fence about getting a Gear VR, then this controller will more than make it worth your while. Now here's a more detailed breakdown..
The Good: This controller makes navigating through menus much more convenient since it acts like a laser pointer or wii remote. Having virtual models of the controller or what it represents in a game is also super cool. Games that use the controller are made much the better for it, as the natural "point and shoot" mechanics feel so immersive, and there are new apps arriving pretty much daily.
The Bad: There is no positional tracking of the controller, only rotational and tilt tracking. Also there is no vibration at all to simulate actions in VR (i.e. shooting a gun). Together, these limit the final immersive feeling of the VR controller. The batteries are also not rechargeable, which I would normally expect from an official Samsung accessory. Finally, you will have to recenter your controller a lot in-app, but it's a single button press process, so you can pretty much ignore this problem.
Final Verdict: (See above) Although this controller is not perfect, I'd rate it a 10/10 for what it is, and the fact that mobile VR is still a new and limited technology at the moment. For $40 this is a no-brainer purchase