Theoretically the generic SHOULD work as well as the brand if the amount of active ingredient is the same. The inactive ingredients (fillers) will vary by manufacturer. There have been anecdotal reports in other threads here of people saying that being switched from one manufacturer to another makes a difference, but that's usually in reference to other medications, not Tadalafil. You should notice relatively soon after making the switch.

One thing to note is that the FDA states "Any generic medicine modeled after a brand-name medicine must perform the same in the body as the brand-name medicine. This standard applies to all generic medicines. A generic medicine is the same as a brand-name medicine in dosage, safety, effectiveness, strength, stability, and quality, as well as in the way it is taken and the way it should be used. Generic medicines use the same active ingredients as brand-name medicines and work the same way, so they have the same risks and benefits as the brand-name medicines."