Other than the quality issue, my experience with the GS150 has been great. I have mentioned before in this thread that the problems are with Pakistani parts' quality and quality control, mostly with body parts. I have not felt/found any problem with the engine or electrical systems yet.
I bought this bike for long distance travel and comfort was one of the most important factors. Out of 7000km, may be 400km was done in Abbottabad, another ~500kms for trips to Taxila, Thandiani and Nathia Gali, rest was trips of over 250kms(Abbottabad-Rawalpindi/Islamabad/Abbottabad) to over 1900kms(Abbottabad-Skardu-Khunjerab-Kaghan-Abbottabad) and a couple of trips to Lahore. Even after an eight hour trip to Lahore, (400kms with three tea/lunch breaks) I am still fresh. It was not the case with Sprinter, which required at least four breaks and after reaching Lahore I was not at all in a mode to ride the bike again that same day.
I am not sure about the power thing though or about the top speed. It depends on one's definition of power(which for a lot of people is burst pick-up or top-speed) or top-speed(which many people consider to be the speed the speedo-meter needle touches, even if for a few seconds). I can say that it is the most powerful bike in Pakistan markets. The top-speed is given in the brochure as 120km/h. Well, practically, the top speed of my bike has been about 105km/h as this was the speed I could maintain for longer than 5 minutes a few times (or 108-110km/h if you consider maintaining that speed for more than 3 minutes constitutes top-speed).
Note: all these speeds were on GT Road, between Sarai Alamgir and Gujranwala. (Dual carriage-way, good road surface and not much traffic.) On single carriage-ways I have never exceeded 80km/h except for overtaking at max 85km/h.