Pro: Very firm (great for P or G spotting), smooth, variety of textures and shapes available, won't dry up lube or your natural moisture like silicone, temperature play options, extremely safe, sterilizable (so it's shareable), compatible with all lubes, hypo-allergenic, boroscillate (Pyrex) glass is very safe, difficult to chip or break and won't shatter into tiny pieces. Contrary to popular fears, glass dildos won't break inside you. Glass toys are also beautiful and many look very artsy and some don't look like sex toys at all!
Con: Firmness isn't well tolerated by some (though many love it!). Rigid, unyielding materials often feel bigger than toys of soft materials. I don't love super textured toys and most glass toys have bumps, ridges, swirls, etc. I can't always find smooth glass toys in shapes and sizes I like. Glass is very heavy, sometimes my hand and wrist can get tired using it. If the external handle or base or whatever is heavy it will either fall out of me or shift out of position if I take my hand away. If you're into hard, deep thrusting, glass may not be for you as it is possible to bruise yourself internally by thrusting too hard with a firm toy.
Although good glass toys are difficult to break, it's not impossible. If dropped on a hard surface, they can crack, chip or break. Usually they will either chip or break into large pieces, not shatter into tiny sharp sliver pieces like a drinking glass or window. It is important to store your glass toys where they won't experience impact, be careful with them over hard floors or other surfaces, and inspect them regularly for any chips or cracks. Any damage renders the toy unsafe and it will need to be thrown away and replaced. Again, breakage is not easy or common and won't happen during normal use but it is possible if they are mishandled or dropped.
The Bent Graduate, Amethyst, Sasha Grey Swell Wand, and the Athena are all good beginner glass pieces. I love my Athena, it was a good first for me.