Hackers can easily use this this vulnerability and take control of your smart phone using a malicious MMS. Devices with android 4.0 and below are more exposed to this attack. An attacker can run a code remotely by sending an MMS which will trick the system to read it as a video then the hacker can do its damage by accessing sensitive data on your phone, camera mic etc.
stagefright detector app
This app can only detect and tell you if your device is exposed to this attack not patching the vulnerability.
The scary part of this Stagefright Detector App is the app anonymously collects data like fingerprints from vulnerable devices. Even though they say they will use it to create patches it doesn't look good for me.
Giants like Google, Samsung and Alcatel have started making patches of this attack.
If you want to be on a safe side for now,
Disable automatic retrieving of MMS messages inside the settings of your messaging app.
Don't download malicious media files.
Even though it is said nobody has been attacked with this exploit yet it is always good to be on a safe side.