To have a smooth experience I would only consider jailbreaking an iP4 or iP4S.
Honestly, once you jailbreak your device, you'll never regret it. Yes, you will have those times where you'll be frustrated and restore your device numerous times to get it just right but other than that, what the community offers is pretty amazing; your phone truly transforms itself into a new device.
However, you need to have self control from downloading every theme, tweak, and app 'cause your phone's performance could suffer from it.
If you search the forums, you'll find a few topic where they talk about which apps are essential and which are nice to have.
Regarding personal information being stolen; as long as you stay away from the repos that pirate paid app, you should be ok.
I personally feel that those repo's (xsellie, ihacksrepo, etc) aren't trust worthy--that could be me acting insecure about my personal stuff, though.