They're pretty terrible, at least on Android. The latest stock android browser doesn't have have a text zoom option, and the text zoom features in chrome or firefox don't really work reliably.
What sucks is Android used to be good, at least in a lot of cases. I don't know how much this generalizes, but on Hacker News I used to be able to pinch to zoom, then double tap the screen and the site would wrap at the zoomed size. This went away somewhere around Ice Cream Sandwich, not sure exactly.