Glass is very firm (great for P or G spotting) and smooth. It comes in a variety of textures and shapes and won't dry up lube or your natural moisture like silicone. It's extremely safe, sterilizable (so it's shareable), compatible with all lubes, and hypo-allergenic. Glass toys are made of boroscillate (Pyrex) glass which is very safe, difficult to chip or break and won't shatter into tiny pieces (it's not like drinking glasses or window panes). Contrary to popular fears, glass dildos won't break inside you. They are heavy, which is sometimes a pro and sometimes a con depending on how it affects your use. Glass toys are also beautiful and many look very artsy, come in a wide variety of colors and designs, with lots of artistic techniques used in their design to create lots of different and unique designs, and some don't even look like sex toys at all!
I love my glass dildo. the heaviness and firmess of it makes it the only reliable g-spot orgasm I can have (without a partner). It also looks so darn sexy! They are usually very reasonably priced too (not to mention safety as mentioned above!).