Although the premise could be seen as offensive, the only part out of the first half I've seen that I'd deem genuinely offendable is the North Korea "Death to America" song that opens the film, portraying masses of North Koreans hating America.
Masses of North Koreans probably do hate America because dear leader tells them to. They believe anything they're told by the government. They are completely brainwashed.
Rogen and Franco did not make up the bit about Kim-Jong Un not pissing or crapping. That is something North Koreans were actually led to believe about Kim-Jong Il, and possibly Un as well. That's the kind of crazy crap North Koreans are taught at a young age, and I bet a lot of them believe it because they don't know any better. They can't go online or watch international media to find out it's all a bunch of bull ****. So is it factually wrong to portray the masses of North Koreans as hating America? Probably not, because they do, even if it's through no fault of their own - they really don't know any better.
I find the plot to assassinate a current leader a bit distasteful but apparently not as much as my son and daughter so I watched it anyway. I remember wondering if the song's subtitles were accurate and it raised my eyes a bit that a movie would have a young girl sing the lyrics. The scene where the three boys were playing guitar and being splattered with blood also raised my eyebrows. I suppose there are dozens and dozens of shows putting kids in worse scenes. I imagine the producer or director must film such scenes in a way that somewhat protects the kids. I guess until I walk a mile in the shoes of the parents, I won't know what I'd do. I expect I'd lean toward not allowing them to participate.
Distasteful? This isn't an elected leader like Barack Obama or David Cameron we're talking about. It's Kim Jong-Un. He's a 21st century Hitler. Go read about life in North Korean prison camps from people who have escaped. So forgive me if I don't find a movie about a plot to assassinate him offensive or distasteful. Someone assassinating that son of a bitch is the best thing that could happen to that country.