I posted a thread on this earlier. In MY opinion, I found jailbreaking it to be more of a pain than a benefit. I don't use Netflix, so I wa shoping a JB might allow me to stream Amazon Prime, but apparently there is no solution for that through Plex or the other interfaces. I also tried some of the other JB features available, but they were spotty and some wouldn't work at all. There are some "free" movie sites that allow you supposed access to tons of free movies, but the streaming sucked, they buffered endlessly, and some wouldn't work at all.
My advice is to simply get a Roku for a measly $60 somewhere and you'll get Amazon Prime and have access to a lot of other channels. I have the Roku and ATV2, and between the two of them, I have an awesome extended media center for my TV. I love ATV, it works great and has some of the features I need that work well with my Mac, and the Roku gives me other options.
Of course, if you just want to give it a shot, you can JB the ATV (I don't know if there's one for the ATV3 yet thought) and give it a try. If you hate it, just restore it like I did. It takes about 20 minutes altogether, not a big deal.