Buck's thermoplastic handles work quite well.
I have a Buck Odyssey 181, ( a very nice knife, by the way) with thermoplastic scales. Based on that, I'd say that Buck's thermoplastic compares favourably to Zytel.
The Odyssey's thermoplastic scales have a G10-like texture molded into them. This texture gives the handle a good feel.
Durability is high. I've dropped my Odyssey a number of times on various surfaces, including tile and concrete, with no problems. Every time I drop the knife I pick it up and inspect it for damage, but there hasn't been any so far.
Rev_jch says that thermoplastic is softer than Zytel. They seem to be around equally hard to me.
I have knives with various handle materials (G-10, carbon fiber, micarta, wood, zytel) and Buck's thermoplastic,with it's good feel, high durability, light weight, and attractive appearance, is good as a lower-cost alternative to the higher priced handle materials.