I had JB my 3GS but my battery life was horrible, then I bought the iPhone 4 and battery life is good again, not sure I want to sacrifice that for a few custom features I would get from JBing?
It seems that Apple has caught onto the JB features and an with some good hopes my only Gripe which would allow us to change icons, and SMS/MMS sounds will be fixed in the next iOS, and maybe even use Songs for Ringtones.
Once we are there, then I will never JB again, however I put 4.0.2 on my ip4 so I am staying here for now
Stock iPhone 4 16GB. I see no reason to JB unless you want to break the law by downloading apps which are against your service providers T&C such as MyWi, or if you want to download chargeable apps for free.
Stock across all iOS devices. I JB'd my iPod Touch 3rd Gen last night for fun, updated it about half an hour later because I found it fairly pointless for my needs. I know it's an issue on my end but everything I installed either crashed or wanted me to sign up for something.