My only experience with Glocks is with a M27, the "compact" .40 cal. It ain't really a small pistol, it just has an abbreviated butt and barrel/slide. It's just as "fat" as a bigger Glock and doesn't conceal for me without printing.
That said, it's reasonably accurate and NEVER malfunctions unless I fail to seat a mag fully or something. It has fed everything I've put into it without a bobble. If mine is any indication fo what others are like, I'd say they are truly reliable.
Oh, and a pard has the larger .40. His is just as reliable as mine.