For startup and application loading it can be an order of magnitude faster (and more expensive), but for map loading it is usually not all that much better. That is especially true of console ports that already optimize loading methods for sequential read, where HDDs are still pretty good at (about quarter to half the speed of an SSD rather than less than 1% in the case of random access)
A good balance between speed and price is to get an SSD boot drive for your OS and things like web browsers, and then a hard disk for large files like games.