Frets on Fire
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Full user review
Not that I have a whole lot to go on since I just got introduced to this game today. So far, as a beginner, it seems no different than Guitar Hero. If you have a halfway decent PC, you should have no trouble playing this game. I didn't experience any of the lag other reviewers noted.
Only three songs is a bummer, but you can always download songs from the original right to it. Using a keyboard as a guitar isn't the greatest experience. Definitely opt to buy a USB guitar.