The first place I lived in China. I still love the city.
First, Changchun is growing. Other cities in Jilin took off earlier than Changchun did, but it's now trying its best to catch up. As in much of the rest of China, "catching up" means bulldozing the old stuff. In the case of Changchun, that's not too much of a loss; mostly brick row houses and 1950's-1960's brick factories (that is to say they're made of bricks...though I suppose they could have produced the things, too). I went back to visit this January and was shocked by how much the city had modernized since 2007.
The area around the train station has lots of great Japanese architecture. Changchun has some very, very sad history
having to do with the Japanese occupation. I don't begrudge them their racism towards the Japanese.
Big Peace World, as mentioned by our inestimable leader, is indeed a fun place to poke around. When I was there in 2006/07 it was the only place in town to get postcards of the city...and the postcards were from ca 1980. They were worth buying (¥5...she started at ¥50) just to see how much the place has changed.
I wasn't really a part of the expat scene there. There's a group called Changchun Friends (website seems to be down and facebook page is inaccessible) that set up quite a few events. There are the usual discos and bars. Many of these were around Guilin Lu, which is near Northeast Normal University.
Good restaurants are to be found everywhere, with an especially strong showing of muslim food. It's probably what I miss the most.
Nanhu Park is a real treasure – a great place to spend a full day on the weekend.
I had lots of very pleasant interactions with people there. Great conversations with taxi drivers, etc. As El Jefe points out, they speak very "pure" Mandarin there...though with a healthy dose of dongbeihua thrown in. My friends from other parts of the country constantly correct my dongbeihua until they realize what it is. Then they just think it's funny.
The winters are hard, yes, but worth it.
I think it's a great place to live...if you're not looking for anything exciting and don't care too much about cultural activities. I made great friends there and love the city for that.