They are absolutely, wonderfully slippery and slick -- the closest thing to frictionless that I have ever encountered in a sex toy material. You may even find that you need no lube at all to use glass toys. I often don't.
Also, before I tried it, I just did not "get" what the big deal about temperature play really was -- it just did not sound that exciting to me. Well, I ended up liking it so much that I now store all of my glass toys inside my refrigerator (safely enclosed within their storage pouches on the highest half-shelf by themselves to protect them from things banging into them). I love to feel how they slowly heat up. If I'm playing with a double-ended glass toy, I play with one side until it reaches body temperature and then flip it around to the other side to feel that side heat up. I love it.
Glass is also rigid and non-yielding, and harder materials better stimulate my G-spot. As a matter of fact, I was unable to find my G-spot until I tried a metal toy (the Njoy Pure Wand) and a glass toy (the Don Wands Bent Graduate).
I also like the fact that Pyrex glass toys can be safely boiled to sterilize, which means that I can share all of my glass toys with my husband for prostate stimulation. (Curved toys, especially those with bulbous heads, work best for both G-spot and prostate stimulation).
And, finally, glass toys are absolutely beautiful. And, provided that you do not drop them and store them carefully, they will last for over a lifetime. And, unlike other types of toys, their beauty will never diminish over time.