I don't know exactly what settings it changes, but the battery profile adjust settings like when data is enabled or disabled and probably the speed of the CPU. The disadvantage to using Battery Saver would be a slower phone with less frequent updates to anything using data.
I tried using the saver mode and everything seems to work. GPS, email, text, voice are working fine. Personally I can't tell the difference. Unless, there are certain options on the phone that are turned off when you are on this mode.
Biggest difference I can see so far is that it lowers the LCD brightness. That and if you switch from "performance to "saver" you get a notice that says "This mode may disconnect your cellular data connection after a period of inactivity". I hope that emails are still pushed through after a period of inactivity.
Edit: This notice also appears when switching from "performance" to "smart".