In practice this is a great idea, but reports have suggested that it could end up eating more data than you would like. In fact it seems that some users are so dissatisfied with the feature that a second class action lawsuit has since been filed which basically highlights the same issues that the first lawsuit did.
According to the lawsuit, it accuses Apple of negligent misrepresentation and also being in violation of California’s unfair competition law and false advertisement law. It claims that Apple should have properly informed customers of the potential pitfalls of WiFi Assist before rolling it out, and that by not doing so the company had purposely misled iOS 9 users.
When claims of data hogging first came up, Apple had come forward and reassured users that WiFi assist is not as bad as they think, and that it only increases the amount of data consumed by a small percentage. However if both these lawsuits are any indication, there are some very unhappy iOS users out there.