Was your Glock NIB or used. I bought a used 17 and it shot far left. I normally shoot fairly well. After having it for about 6 months I decided it wasn't me... I had 3 dot night sights installed similar to those mentioned earlier in this thread and low and behold the Glock shot MUCH better.
If you had the gun to a friend does he/she shoot to the left as well? I did have one advantage when I did this the person I handed it to was a former US Army Pistol team member. (I feel very inferior when I shoot with him)