I came to New Zealand expecting adventure, and was not disappointed. Dunedin is situated so that within 4 hour drive you can reach almost all of the lower half of the South Island. Otago, Canterbury, the West Coast and Southland are all open to exploration even just for a weekend. As an avid backpacker the tramping tracks here are unparalleled by any others I have done in the world. If you are coming to New Zealand to see the country then the University of Otago is the choice. After travelling most of the country (including the North Island) my personal opinion is that national parks on the South Island are the most beautiful. Fiordland, Mt. Aspiring, and Mt. Cook National parks are where I spent most of my weekends.
Even when not travelling Dunedin is a fun city, albeit slightly on the cold and wet side during the winter. The University Flats will likely put you into contact with many like minded international students. It will not be hard to find a good group of friends to go on adventures with. However it does take more work to get to know the Kiwis well. As an international student I found them friendly, but also fairly uninterested in becoming friends. While this was expected, it is was difficult to get entirely immersed in a kiwi lifestyle when you aren't living with them. Kiwi hosts (a local that lives with you in your flat) are great and very helpful.
Academics were different from my home university but not overly so. For the classes that I took (Humanities type classes) the main differences were grading. Most of the weight for the semester comes from the final and a mid-semester essay. Classes were engaging for the most part and generally not too difficult. I cannot speak for more specific science classes however.
Overall you cannot go wrong with this school. It caters to many different interests and you can find something to do almost every weekend. Fed up with going out to town? You can easily drive an hour north and camp on Long Beach and go surfing. Surfing isn't your style? Head to the mountains and go skiing and hang out in Queenstown. Want to get away from civilization? Drive an hour north of Queenstown hike for a day into a hut and you will find yourself about as remote as it gets with only alpine parrots for company. Pairing a good academic school with opportunities to see a lot of what New Zealand has to offer is exactly what the Uni of Otago does.