I tried to use this coupon today and they had a copy of it at the register saying that it wasn't a valid coupon. So YMMV I guess.
On a good note though I was browsing thought the used games and I had the strangest feeling to look though the GC games for Ikaruga, and voila it was there! only $15.99 too! I didn't get my 25% off, but I'm still happy.