Hey! So I'm a graduate student who just began this fall and I feel as though I can at least answer some of your questions!
1. I think the city is great. Then again, I'm from a small town so I'm used to the atmosphere. There always seems to be something going on, though, which is nice! And there's a decent bar scene in downtown Champaign to keep yourself entertained. I haven't had much time/money to try out any local restaurants, so I can only tell you that the grocery store situation is good as well - there are plenty of options, from big box grocery stores (i.e. Walmart, Meijer, etc) to smaller, local, and even specialty grocery stores.
2. If it's just you, I recommend sticking to the outer perimeter of campus, but try to find something on or close to a bus line (Check out cumtd.com to find out more about the buses). If money isn't a concern, then you will be able to fork over some cash for a parking pass (~$600) for your car - otherwise, the bus system is good enough to get you where you need to go. I live in Champaign and I love it - I am walking distance to downtown and right off of a major bus route. But I do know a lot of graduate students live in Urbana, as well. I think it really depends on what you can afford.
3. The people seem nice. I haven't had any issues yet. We're in the midwest, which is known for it's small-town charm and friendly people.
4. I'm 25 and I'm in the middle range of most of the people I know - however, that's because most of the people I know are new graduate students, as well. There are plenty of graduate students (therefore your age range) at this university, but there is also a very large undergraduate population. You will learn that there are undergrad hangouts and grad/young adult hangouts, so there is very little to worry about in terms of finding people around your age to associate with.
5. You will be about 2.5-3 hours drive to downtown Chicago, so it is definitely do-able in terms of a weekend getaway.
Let me know if you have any other questions! And good luck!