It really depends who you're aiming at getting business from. You'll get nothing from large companies because they'll work to ISO 9001 and will require their supply chain do the same. Mid-sized companies will likely also exclude non ISO 9001 as often they're in the supply chain for a large company.
I've never gone through it myself but I hear it's not painful. The general gist I've gotten from talking to people who have worked through it is that it is almost "decide how you're going to operate, write it down, stick to it".
You might get away without it if you're making end product (tools, your own products, repair work, etc.) but not if you're making components for other companies products.