I only own one glass plug, the Sexy Spades Large. It feels very nice, but it makes its rigidity known after a short while of wearing. When it comes to plugs, I would prefer silicone for its softness, or a lovely stainless steel Njoy plug, as they are the ultimate in pluggy comfort.
My biggest concerns with glass plugs lie in the base; it is pretty rare to find a glass plug without a circular base, which I find uncomfortable after about half an hour. There are interesting ones from the Pipedream Icicles collection with a ring base, but I'd be afraid of breaking it if I were to sit on a hard surface.
All of that being said, I do love glass as a material, I just prefer it in dildos and probes instead of anal plugs.