I just saw this vídeo about Google Now, but I don't have the same behavior on my Motorola Razr D3, with Android 4.1.2. I got the search results, but with no voice reply.
Is it the voice reply exclusive for Samsung phones?
Voice reply isn't exclusive for Samsung phones: I get it with the same "what's the weather" query on a nexus-7. But it's controlled by a setting in Google Now. From the main window, go to the menu and choose Settings. From the settings page, you want Voice, then Speech output. Set it how you like.
But take note that you only get a voice reply for the special queries that Google Now is programmed for, not for web searches. To get a list of these special queries, press the microphone icon and ask, "Help."