michinyon:
I have used it and find it to be pretty accurate.
It is fine if your vehicle has limited actual acceleration.
I suggest you try this. Put you MPU 6050 in a car, and drive rapidly in a circle as fast as you can.
If you are driving counterclockwise, the vehicle will be continuously accelerating to the left. The little
weights in the accelerometer will be pushed to the right, and you will get the apparently direction
of gravity there. See if you teapot stays upright, or not. It won't.
It does, I am using an MPU-6050 and I attached an HMC5883L magnetometer to the auxiliary pins, and I am running a MARG orientation fusion algorithm that uses the data from the accelerometer + gyro + magnetometer.