The ZTE Open is basically a smartphone for people who don’t really need most of the bits of a smartphone. If you want to just check the weather report, maybe look up a factoid or news story on the go, the ZTE Open is a useful and economical way to do that, while still being able to take advantage of those cheap prepaid plans. For the more tech-savvy people and smartphone enthusiasts, it’s probably too limited to be worthwhile even once the software side catches up.
Then there’s the separate issue of the Firefox OS. Right now, Firefox certainly isn’t bad for a first release, but it’s going to need to seriously beef up the availability and quality of apps if it wants to compete with the major players.