i've heard that some would eat through metal filings if they could, but mine never has...granted he's come a little too close for comfort to one of my biggest mushrooms but I just move him and he's fine. Mine doesn't even spore or do any of that mess, he's fantastic. Especially when he cleans the glass.
At first I hated him because he came with the tank and I really didn't like him, but he's grown on me. He's really fascinating and I've heard its hard to get them as big as he is. But thats just my experience, I didn't really choose the urchin but compared to the black and purple ones I think the pencil still looks the coolest as far as shape and color go.
-Allison