pro's.. you can customise your iphone. themes, ringtones, tweaks.
Con's.. not much, it might crash once in a while.
There's also the pirated software with or without viruses, or stuff that's too hot for the app store (including stuff that competes with Apple).
The real reason why I do it on my iPod is to expose the underlying filesystem (and God forbid be able to use a filemanager), and to use unix/linux/bsd tools. iOS is basically a BSD with a slick interface... If nothing else it means that you can use a filemeanager with SFTP to treat the thing as a big ass USB disk.
Oh, just make sure you have a hardware / OS combo that you can do an untethered jailbreak on. Otherwise you'll have to plug it in every time you cycle the power fully on / off if you want the jailbreak. Before you ever update your iOS always make sure that there's an (untethered) jailbreak for it.
Oh, SBSetting (or whatever it is now) is pretty handy as well...
Android, iOS, and BB10-- You're all dealing in various degrees of 'less free as in freedom' walled gardens, regardless of the amount of source code available or not. Though the former is more free and the latter less.