18K gold is a good choice if you want a piece of jewelry that is reasonably pure but is not overly soft. This alloy, along with 14K gold, is a good compromise between durability and purity.
Of course, 14K gold is harder and more durable. However, if you are very sensitive to nickel, even some 14K pieces may give you a rash, whereas 18K gold is much less likely to cause an allergic reaction.
It is not a good idea to choose 18K gold for pieces that you will wear every day, especially rings or bracelets, which are more likely to get hit or scratched. In such cases, 14K or 10K gold may be a better choice as it will not bend, break or scratch as easily.