Rails: Best practice to store user settings?

If you use PostgreSQL, the best solution is to use https://github.com/diogob/activerecord-postgres-hstore/. It's a simple, fast and reliable way to store hashes in the database. Since it's not just a serialised text field, you can search on it also, and you don't need to create a new table, as in HasEasy.

def User
  serialize :preferences, ActiveRecord::Coders::Hstore
end

user = User.create preferences: { theme: "navy" }
user.preferences['theme']

The "open" table approach makes it difficult to model with AR, since you have to worry about the types of the data (boolean, int, string, etc). I've always added prefs as columns on the users table, and then move them out to a user_preferences table if there are "too many" of them. It's simple, and it's easy to work with.