The generic blue 120A ESCs work great...if doing only 2S. If exploring the 3S realm, I warn you of the weak internal BEC. I've burnt out (fried) two of the ESCs doing so. I had to use a Castle Creations BEC with the blue ESC to supply power to the receiver/servo. Other than that, their are fine for lite duty.
Another note: The low budget blue 120A ESC only puts out 5v on the BEC. To get full potential from a 6v servo, running an external BEC would be your best bet. For most people, running a servo at 83% power with internal BEC is acceptable.