All my iPhones have been jailbroken (even the first generation) and I honestly could not imagine using them without it. The main benefits were facetime over 3G, tethering, and a few other tweeks. Winterboard and themeing in general, I found, introduced a lot more instability and can cause the springboard to crash and go into safemode. Widgets also will naturally drain the battery faster as they are running in the background. You just need to be careful about what you install. Widgets on an iPhone are really meant to work on the lock screen. It is possible to have them on main screens but you just have to do so many tweeks in order to prevent the widget from covering up icons; it becomes a real hassle. Also, a lot of the widgets need to be updated manually. Such as weather widgets.
In general, jailbreaking will improve functionality and make the iOS experience better but themeing on iOS is nothing like Android. The learning curve is large and functionality is not nearly as polished. Out of all the mod's theming WILL drain the battery the most. I decided themeing was just not worth it on iOS and my main apps were MyWi for tethering, My 3G to prevent the wifi only limitations, fakeclockup to speed up animations, sbsettings for easy on/off functions, and a few other ones here and there.