I am a University of Cologne student pursuing my PhD at genetics institute in Cologne :) so I feel qualified to answer this question. Cologne, it's beautiful. I want to tell some specific points here

• It's one of the largest cities in Germany yet it feels calm here. You don't see tall claustrophobic buildings, uber fast life, moronic traffic. It's balanced and comfy city. It retained Roman touch so it's vintage and yet modern.
• People here are very friendly. Almost everyone can speak English so travelers or foreign students don't face trouble to communicate. City has many young people which gives it a fresh feel. You wouldn't find people giving racist remarks. Cologninans are the best.
• There are lots of cafes, restaurants, bars here keeping the city chilling. Best part is, you don't need to be rich to enjoy clubs. Students like me can visit bars, clubs easily without a dent. Student clubs are famous for this.
• There will be one or another thing going on all the time in the city. Flea markets, concerts, live music you name it. You'll not be bored. There's always something you can do.
• Cologne has one of the largest universities in Germany. This allows student influx to this city marring it very international.
• Rhine, the river in Cologne is one of the most beautiful spots in the city. It's well maintained, you can take a long walk or rent a bicycle here and go as far you would like.
• Being a student, one of the best thing is you can travel in the tram for free using your student ticket. After 19:00 you can even take one person with you for free. Now that's called mobility. This is so awesome. Not only in the city, but as a student you are free to explore the NRW region. Like seriously Cologne, that's amazing :)
• City is very relaxed on having different cultures. You see Asians, Turkish, Hispanic, Africans and who not. And oh ya Germans :)
• Food here is plenty. Wait, did I already say that? All cuisines, all prices

I can go on and on but you know what I feel. I love this city.