Problem with lightning:input component. Onchange event

Thanks for bringing this up. This is a known problem that is currently being worked on. In the meantime, you can work around the issue by binding a change handler to the value attribute:

<aura:attribute name="value" type="String" />
<aura:handler name="change" value="{!v.value}" action="{!c.onValueChange}" />
<lightning:input type="text" value="{!v.value}" />