Tried it, not worth it
I noticed a definite decrease in battery life and stability after I JB my iPhone 4. I had SBSettings installed, Winterboard, and another minor tweak or two. Weird things started happening like the Reminders app icon showing up like the Notes icon, and then when I respringed it, it would work again (reminded me of Windows machines - reboot fixes everything). Also, a game I love to play, Carcassonne, was noticeably slower.
Since I don't switch on and off Bluetooth and WiFi very often, it's really not worth it. A lot of the tweaks are so marginal that I'm fine with the Apple version of things (No, I do not need to read FB or Twitter from my lockscreen. If I'm going to read them, the apps work great for me). If I needed a mobile hotspot more, maybe I would keep the JB. That said, it was easy to do and easy to reverse, so if you want to try it out, go for it.