Speaktoit is available on several platforms, including iPhone; Robin is exclusively for Android phones. Consumer Reports asked each similar questions. Both Robin and Speaktoit do well with some requests. But they struggle with other questions. Siri, on the other hand, has no problem with these requests.
Consume Reports says Siri remains the voice-recognition standout. It's hardwired into your iPhone and has access to much more of your personal information. And that's an edge that none of the other apps can compete with yet.
If you don't have an iPhone with Siri, Consumer Reports says downloading a free app gives you at least some voice-recognition capability. Speaktoit seems to be one of the best so far. It works on both the iPhone and Androids.