Another note - it won't stop you from manually checking for mail and such - you can go into mail and press the sync button. The phone will act normally, as far as the phone is concerned, but it will act as if it doesn't know there is a data connection at all, unless you have the screen on and manually force it to check things. But as a side effect of that, Live Tiles won't update. So if you are in battery saver, and you have a tile with notifications, you tap it, you can see whatever it is counting (a game, for instance, telling you how many "turns" you have) - if you play those turns, or whatever (or the WPCentral app - unread articles counter, for instance), even though read, the tile will not update.
And like WPmunkey said, it is handy if you know you won't be able to charge for a long time. Put it in battery saver mode, and you'll be amazed at how long it will last.