Congratulations on your ride I love GS150. The fact that its the ONLY 150cc bike which actually makes sense (because of the engine) makes me respect this product. And yes, the quality control issues are still there, but really nothing deal breaker. Its not like you are gonna face problems in it as your daily driver. Just take good care of it, and the bike will return the same effort.
As for your breaking issue ... you "feel" satisfied because you have come from Sprinter, which is a relatively small bike, and does not have that much mass as a GS150. Once you get a hold of your bike, you will notice that these brakes are not "brakes". They are a "nightmare rolling down the road, waiting to happen". I hate drum brakes in general. The fact that they are useless once you go above 60-70 km/h ... YOU ARE DONE. Phir aap ko ALLAH he bacha sakta hay.
So if you have some cash left, i would really advise going for a Disk Brake. If you have operated a Disk Brake in the past, the "Response time" alone will be enough factor for you to move to it.
Also, if possible, get its Rims changed as well. Get better quality ones.