I bought a Hubsan X4 just to learn to control a quadcopter before trying out my own. This is what I learned:
1: Difficult to calibrate
The controller was difficult to program. Almost no feedback shown if what you did to calibrate was correct or not. With my quadcopter I'll obviously understand anything I do so... :9
The controls are extremely sensitive. The very first flight happened completely by accident and ending with the X4 falling down behind a heater. Awesome. However, after a couple of flights I got used to it.
3: Hover is difficult
It took a couple of flights to get it to hover at roughly the same altitude. It should be simple to do. Press the throttle forward carefully until takeoff. Then back it down a little when it has reached the height you want. Then if it goes down, then throttle on again etc until an equilibrium has been reached.
In the beginnning it was really difficult because the engine controls had to be further "trimmed". I.e. I had to manually counter any sideways movement that it had using the trim controls on the transmitter. If I only press the throttle it should theoretically just go straight up. That has happened very rarely - and it make sense if one of the engines just turns slightly faster than the others it will cause it to go in a certain direction depending on which motor is running faster. This has to be countered quickly by a PID-based control system to lower the speeds of selected motors. Another problem is that the battery seems to slight out and in of it's compartment severely skewing the balance of the craft.
One benefit I hope I will have is that my quadcopter till be a lot heavier than the 50-something grams of X4. Hence any tiny difference in the propellers should not have an as large effect since the propellers need to move a larger mass.
In addition, the IMU I've bought also comes with a built-in way to measure height. So maybe a better way to make it more stable would be to also control the throttle using the height.
4: Prepare to crash
So far it has fallen down behind a heater twice, crashed into the 4m heigh ceilling - once crashed straight down and another time bounced a bit and then continued full speed ahead into the corner of our bedloft. Apart from the propellers the craft doesn't really seem to take any damage. But I definitely need spare propellers and as soon as I go beyond my square wood frame I need to find a clever way to protect the electronics. Maybe I should even buy some protectors for the propellers.