I gunked up my iron with iron on interfacing, and my iron has teflon soleplate so it can't be scrubbed. I tried the manufacturers directions to clean it, and it didn't work.
I bought Sole Plate Cleaner at WalMart, and used it twice - still not clean. I ended up making little scrapes on the iron using the edge of a knife. I guess I could have tried GOO GONE, but I ran out of patience.
It beat having to buy a new iron, but I was really tempted to get a new one if I couldn't get the black goo off it.