The Android Wear devices currently available let you respond to texts as well as read them – but they employ either voice-based input or canned responses to do so, and neither is entirely ideal.
I have enough trouble typing (still!) on an iPhone screen. This may make it a little easier but the constrictions of a watch interface are still going to make this difficult. That’s precisely why I think Google has pushed voice commands so hard for these devices; no it’s not entirely ideal but the dorkiness of talking into your watch outweighs the inflexibility of a form factor that makes it damn near imposible to type unless you have small fingers. Even with assistance from something like Minuum. Look for this to be a continuing issue with smartwatches and wearables in general.
Nice profile on what apps have started building extensions compatible with Android Wear.