I lived in Istanbul for about 10 years (then moved back to SF; currently living in Boston).
Pros:
* Istanbul is an extremely large city, pretty much like LA, so chances are there is something to do for everyone. Your favorite band may show up some point within your life time, you don't feel isolated from int'l community.
* Weather is pretty nice, not too warm, not too cold, you experience 4 seasons. But it's a little too warm for me, but I like it a lot colder than other people.
* People are overall pretty great imho, although certain neighborhoods can be worse, too conservative etc, but overall it's fine.
* There are certain places, e.g. Prince Islands, that are simply gorgeous and I'd love to spend rest of my life there in peace.
Neutral:
* City is pretty secure; not like super secure that I'd go out alone drunk at 3AM without thinking, but I think it's still more secure than cities I lived in the US (maybe except Boston?).
* EDIT: I should also add that it's a very crowded city. It's densely populated and about ~15 million people live in "the city" which is like this multiple set of downtowns, very close to each other and span two continents. You might like this but I think this makes everything very messy.
* EDIT: Nightlife is pretty ok I think? I'm not a huge club person, but I've visited many bars (especially those in Kadikoy) and I really miss them, there was something about them.
Cons:
* Not very tidy, clean like European cities. I think it's still orders of magnitude cleaner than LA or NYC but e.g. compared to Boston, it's less nice. Architecture etc are pretty banal. Living spaces are usually like condos and nothing exciting.
* Political climate is as bad as it can possibly be. Although some parts of Istanbul are extremely liberal -- feels like just another European city -- Turkey is currently a very fucked country, led by a dictator, and I personally wouldn't live there at the moment. But you know, politics change.
* Not a very developed tech scene. I know some interesting startups and was able to closely work with some of them; so there is definitely some interesting stuff happening, but absolutely not like any major city in US or Germany.
* Everything is extremely unstable, Turkey is "developing" very fast e.g. what is a gorgeous park today can end up being a horrible tourist hellscape a year later because economy relies so much on tourism. Everything is in flux, and things are done without thinking about aesthetics etc. Almost everything is half-assed. E.g. Taxim Square was a really nice and historic place when I was there, but somehow they managed to rebuild and fuck it up last few years.
* Bad traffic, but that goes without saying for most major cities. Drivers are definitely bad, but like you know, you adapt.
* EDIT: A majority of people are cigarette smokers. Not 420 friendly (since cannabis is highly illegal) but it's possible to find it if you know a guy who knows a guy.