As Mystic already said, it's 100% legal.
As for the 'problems', I've been jailbreaking iPhones (my own and friends' and family's) since the first iPhone came out, and have yet to experience an iPhone that couldn't be restored anymore. I've installed and/or tried just about any system tweak that exist, and I still had no problems. If you only use the 'official' repositories to install stuff, you're pretty safe from any problems. There is certain stuff that can slow your phone down, but that can generally be easily removed if you encounter it. Personally, if I couldn't jailbreak my iPhone anymore, I wouldn't even buy it anymore. Jailbreaking allows you to add A LOT of functionality, with little effort. People that mess up their device, usually do so by installing crap from 'unofficial' repositories. And still, a restore (from DFU modus) has always been possible in the rare case anybody messed up their phone.
I've also been very active on some Dutch iPhone forums, where I used to assist people in jailbreaking their devices. In all these years, I've never encountered even 1 person who wasn't able to fix their device themselves, with some guidance. So yeah, I'm always a bit wary when people say they messed up their iDevice beyond repair, because of jailbreaking. Because I simply can't see how they did.
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