It's impressive, but a fairly standard technology for launchers to have. The Space Shuttle did the same thing five times: main engines would ignite at T-6.6 seconds, and there was a brief window until the solid rockets ignited at T-0 when sensors could safely shut down the main engines. Four times it was done at T-3, the remaining time at T-1.9: http://en.wikipedia.org/wiki/Space_Shuttle_abort_modes#Space...

But once those SRBs were lit, it was going up, or blowing up...

While it's not a capability unique to Falcon, it's still neat that they're able to be sure every engine providing thrust is operating normally before they let it go.