-Reliable. The gun has had 0 malfunctions
-Accurate. The longer sight radius helps when targets get small or range gets long.
-Durable -Easy to maintain
-TONS of aftermarket support (not as much as some models however)
-Accepts 17 and 33 round factory magazines
-Glock's customer service is excellet
-Cheap replacement parts, should you ever need them
-Low Bore Axis combined with relatively long slide resulting in a soft shooting gun.
-Easily modified to fit the shooters' preferences
-Requires very little cleaning/lubrication to run reliably
-Many of the worries about brass to the face are assuaged; I've never heard of it with the G17Ls.
-Increased velocity due to the long barrel
-Not offered under the Blue Label or GSSF programs-Factory adjustable sights have a cheap feel
-Gold Dot +p 124gr: 1331 FPS, 487 FT/LBS energy
-HST 124gr: 1203 FPS, 398 FT/LBS energy
Here's a video with some shooting from the 12m line, a size comparison, and a chronograph test with some popular carry/defensive loads: