Living in Chiang Mai has many aspects, and what it's like will be as much about the person doing the living as the place being lived in. My guess is the question is more like "what is it like as a foreigner to live in Chiang Mai". Here are a few aspects that come to mind.
Language: if you don't speak Thai, it can be frustrating, but a little Thai can go a long way. The local people in Chiang Mai are generally friendly and easygoing and so basic stuff is quite low-friction, even with minimal Thai. That said, this is a Thai culture and the Thai language is vital to understanding what is going on. English is not widely spoken by most Thai people. Learn a little Thai can help, learn how to read Thai and life is much, much easier and better (such as communicating with the local Thai people).
Costs and Availability of Goods: Chiang Mai has great prices on a number of things, including rent, food (local markets and local restaurants), even things like DSL Internet and cell phone service are less expensive than in the US. Some things are more expensive, such as computers and cell phones, though only by about 20%. Camera equipment is more expensive than off Ebay and Amazon, but many Ebay sellers offer International shipping for free, and usually ship out of Hong Kong. Some things are difficult to get, and/or need to be imported, such as books, kindle readers, even an Aeron chair was cheaper to buy on ebay, ship to Thailand, and pay VAT, than to get one from Bangkok. There are three Apple stores and an Apple Care location (in Kad Suan Kaew), prices are slightly more than in the US. Regarding foreign food, there are enough foreign restaurants and grocery stores to cover most items desired. Overall, cost of living can be 2-3 times less expensive (depending on lifestyle).
Transportation: Chiang Mai metropolitan area is not that large and so getting around is fairly easy. There are a variety of transportation options: tuk-tuks, songthaews (trucks), taxis, bicycling, motorcycling, and cars. There is not really a bus system here, but most transportation is fairly low cost. The traffic can be a bit hectic and some foreigners shouldn't try and drive in it (I know many Thai people who won't ride a motorbike in Chiang Mai because of the pace of traffic). However, this is nothing like Hanoi or Ho Chi Minh or Bangkok.
Climate and weather: It gets hot in Chiang Mai. Most foreigners not from a tropical location will suffer from the heat. The ways of dealing with it is to follow the Thai people. Avoid being out in the heat and direct sun. Wear clothing that covers the skin, including hats, jackets (even mittens) and use umbrellas/parasols. Also, just stay out of the heat during the day. Stay inside with the air conditioning like all sensible Thai people! Have to go shopping mid-day? That is what Malls are made for! Don't exercise mid-day (only before dawn and near dusk during the hot season).
Pace of Life and Events: Chiang Mai and Thailand in general people work hard but they aren't so serious, and most enjoy family and spending time (especially eating meals) with friends and co-workers. Food is an important aspect of Thai culture. Another is Thai Buddhism, and most Thai males are expected to spend some time as a monk at some point in their life. The daily feeding of monks who make their rounds, the lunar weekly "Buddha Days" and the Buddhist calendar of various festivals and events, combined with the worship at various (Buddhist and non-Buddhist) shrines throughout the area are a part of the pace of life. For the large celebrations there are days-long festivals which are very important and combine the importance of visiting and revering parents and family as well as having fun with friends and neighbors. The main thing is to smile, have patience, and try and get along. People from bigger towns and more aggressive cultures might get annoyed, but coming from Honolulu where I lived for 7 years previous to moving to Chiang Mai, I found it to be a similar size and pace of life.
Expat Community: There are various kinds of expats in Chiang Mai. Many should be avoided, especially the retired bar-dwellers and the so-called digital nomads. However, there are many interesting long-term residents who can be interesting to know, as well as some of the return visitors who come every year.
Work: There are a more limited number of jobs available, as work permits are complicated and are created by organizations who specifically need foreign workers, mostly around teaching English. Entrepreneurial foreigners can figure out how to create and grow a business in Thailand.
Health: There are some health risks in Thailand, such as the increasing air pollution (very bad in Feb/Mar with burning of the fields), dengue fever (I've had it once in Chiang Mai, Thailand, once in Honolulu, Hawaii). Crazy traffic. That said, there is very good, inexpensive healthcare. Good hospitals (also some mediocre ones) are very affordable and one can find reasonable health insurance. Something the west simply no longer has.
Education: In Chiang Mai there are several universities and private schools. It is something of the education hub of Northern Thailand. For people with children there will be an additional expense of private schools, though unless one goes for the full-on International schools, they are much less expensive than private schools in other places. Some complain about the level of education in these schools but there are few places in the US with excellent education and some places are much more dangerous.
Crime: There is a degree of crime though most of it is constrained to certain areas. There is some amount of Yaba/Speed/Crank, prostitution and other issues. Mostly my experience is that it is safer to wander around Bangkok or Chiang Mai early in the morning than places like San Francisco, Honolulu, or Seattle. That isn't to say that foreigners are completely safe as in certain areas (Phuket, Pattaya) they are seen as targets by a criminal element. There is simply much less of this element in Chiang Mai, though certainly one can provoke a bad situation if you go into an area where Thai men are drinking heavily. Especially if there is a lack of respect shown.