It's not half so bad as the crack I've just discovered in the 26" rear wheel of my hack MTB, which I am now keeping a VERY close eye on to see if it gets any worse, with a view to swapping it out. I suppose it depends ultimately on your attitude to risk, and how far and fast you ride the bike it belongs to. Mine is only ridden locally and at relatively low speeds, so I'm not panicking over it.