Are you talking about the regular power saver, or extreme power saver (EPS)? I leave the normal power saver on, which saves some battery with next to no performance loss. I only turn EPS on if I'm really low and don't expect to have enough juice to last until I can charge it. It really extends the battery life tremendously, but also makes a lot of trade offs to do so. Mobile data, Bluetooth, and Wi-Fi are restricted, can only run a few system apps, screen is dimmed, etc. If that's not something you can tolerate, try manually turning mobile data off and leave it unless you actively need it, then turn off again afterwards. That and leaving the screen off will save a ton of battery life while still allowing full function of the phone.