My opinion of GMC can be summed up with a long list of foul words.
I had a 1995 GMC Jimmy. Drove it for 5 years, had ~100,000 miles on it.
Good thing I had the full warranty, because for the last 2 years I had it, it broke down EVERY MONTH on the dot!
It wasn't small things either - stuff like the alternator, transmission, brakes, timing belts, etc.. all took a dump at one time or another.
The service wasn't that great either. Ther service department at every GMC shop I went to overcharged me for everything, jerked me around, and were penis-heads in general.
I'm sticking with my Toyota. Awesome car, awesome service, never going back to American.
Interestingly enough, my Tundra is the only full size pickup to be completely assembled in the 'states. I bet GMC can't boast that.
*edit*
In GMC's defense, I will say that the 1995 Jimmy was the first year with the new design and engine, so that may have accounted for problems. However, one of my coworkers has a 1996 Jimmy and hates it more than I hated mine, for the same reasons.
I know that GMCs in the past were very reliable but it seems that in the past 6-7 years quality has started to go downhill, that's just my observation.