A 1911 labeled a GI is typically going to be Mil Spec, in that it has looser tolerances and more closely adheres to the original specs handed from God himself to Saint John Moses Browning. Looser tolerances means that it will run more reliably, but may have a heavier trigger pull, slightly decreased accuracy and less pretty shit hanging from it than your typical off the shelf "semi-custom" model 1911.
Para's are good shit, some of the best competition shooters out there use them. One of the first 1911's I've ever shot was also a Para. Like Austin said, I own a Rock Island Armory. If you can find one, they are some of the best GI's out there for the money, but have been coming up enough in price that it's no longer worth ordering one instead of grabbing a Springer GI off the shelf from anywhere.
One day I'd like to own a full out custom 1911, but for a gun to actually beat on and rely on I'll take a GI any day.