A point-of-view video of the world's fastest roller coaster has been posted on AMEinfo.com. Formula Rossa launches trains at 149 mph, taking the record from the not-really-ever-opened Ring Racer at 135 mph, and before that Kingda Ka at 128 mph.
The video makes the ride after the launch appear slow, but I'm wondering if that's just because of the massive scale of the ride. If I did my math correctly, Formula Rossa covers its track (launch to hitting brakes) at an average speed of 63 mph. It's not a perfect figure because of taking out the length of track between the brakes and launch, but it's a good estimate.
For comparison, using the same technique I found that Xcelerator at Knott's averages 62 mph, Storm Runner at Hersheypark 59 mph, and Sky Rocket at Kennywood about 31 mph. So that's still a pretty speedy ride that Formula Rossa delivers.