I am not in any way an expert. I am answering based strictly on my own personal experience. So here we go.

I wouldn't have ever considered myself addicted to cocaine. But I have spent countless nights awake shooting up coke in my past. Despite this, I was mostly addicted to heroin. Cocaine was more of an occasional thing because people would give it to me or it was around so why not. The difference is that cocaine is more of a mental addiction and heroin is more physical.

When you do coke, it just makes you want to do more coke. You just keep doing more and more because you don't want to come down. Coming down from coke is hell. Unless you have a buffer like heroin. I used to line up my shots depending on how much coke I had so that I would start with pure coke at the start of the night and gradually decrease the coke and increase the H until my last shot was pure H and I could sleep off the cocaine without feeling bad. The next morning, I would be dope sick but have no desire for more cocaine. That's how it was for me. Keep in mind that cocaine also comes in many forms and most people don't actually inject intravenously as I did. I cannot speak for the smokers and snorters though. I was strictly IV.

The good thing about cocaine vs heroin is that you CAN stop. You don't have painful physical withdrawals like with heroin. In fact the worst part of using coke for me was actually using it. It isn't even a good feeling (for me) but it tricks your brain into thinking it is so you don't want to stop. Days pass and you don't even notice. You will spend every penny you have on coke because the minute you start coming down, your brain goes into autopilot so you don't care about anything else or want anything else or even comprehend that anything else even exists. Cocaine is not a means to any end except more cocaine.

If you think you might be addicted to cocaine, please get help. Even if it is just a friend you trust who can watch over you for a few days and make sure you're okay. I quit heroin alone on someone's couch with nothing and nobody to help me. I cannot even begin to tell you how that feels.

On the other hand, if you think someone you care about may be addicted, it won't be easy to help. As an addict, I can tell you two things:
1. The president of the US could have told me I was an addict and I would have told him to bugger off. I didn't believe anything anyone said about me. I had to come to the conclusion on my own. And I did... eventually. It took months of heavy use before I finally admitted I was an addict. It gets worse though because...
2. Realizing you have a problem is not the same thing as being ready to fix that problem. I knew I was an addict for years before I cared to do anything about it. And nothing anyone could say or do was going to change that. The day I made the decision to get sober was not at the fancy rehabs my parents forced me into or during the various treatments they threw their money into. It was just a random day when I decided I had enough and I knew it would stick because I went cold turkey. I felt everything. And I never want to feel that pain again. I know the price of using. It isn't worth it.

So what does it feel like to be a cocaine addict? I guess it depends what stage you're in. Denial, realization, acceptance, not ready to quit, ready to quit. I don't know if everyone goes through those same steps. But if a person is in any but the last stage, you cannot help them. Nobody can. Only the addict can help him/herself. When he/she is ready and asks for help, be there for them. But be wary because addicts are tricky. They might lie to get something from you. A friend of mine used to joke about how rehab was a vacation for addicts (for those of us with wealthy parents). We could go get high until we were so strung out and couldn't take it anymore and then tell our parents we are ready to get clean. A good rehab is like an all inclusive resort. They give you meds so you don't feel sick and they have activities and gourmet chefs and they take good care of you. You can spend a month there with your family footing the bill somewhere between $600-800/day for a decent place and then you leave there refreshed and ready to start another run on the streets. Addicts don't even realize how many people they are hurting. Or they choose not to. Eventually, you come to a fork in the road and both paths lead to a dead end - choose drugs and eventual death or choose immediate death (suicide). I remember the day I found myself there. It was on a bridge and I had made the decision to jump but couldn't gather up the nerve. So I eventually turned around and walked away.

Addiction hurts a lot of people. I assume that you ask this question for a reason so I will add one final thing: if you need someone to talk to (someone anonymous, experienced, and nonjudgmental), feel free to message me privately. I've been sober for over 6 years. I've seen and heard and done a lot of things. And now all I can do is try to help other people who are suffering the way I once was. No matter the situation and the choice you make, I wish you well.

*Update* Since this answer was moved to include crack, I want to add a little about my experience with that. As I said before, I only ever shot up cocaine so I never personally smoked crack but I have lived with a group of crackheads and it is absolutely terrifying. Crack turns a nice, normal person into a complete monster. It makes them totally paranoid and crazy. The people I knew usually roamed the streets at night, running inside abandoned apartments to smoke, then going back out in search of more crack (or money to buy more crack) until the morning. I was squatting in an apartment with maybe 6 or 7 other people who all smoked crack. I was the only one who did not. I would often be on the couch, trying to sleep while they smoked in the corner. Many times, they would wake me up to ask me for money or something I could give them to buy crack. Many times, they would wake me up to try to force me to smoke with them. Or they would somehow convince themselves that I was hiding their crack and/or money and would harass me. Or claim that I smoked their crack even though I would never touch the stuff. Once, a girl tried to trick me into smoking crack by telling me it was something else and she wouldn't tell me what it was so I didn't do it. She was disappointed. Not sure why they would want me to do it. Maybe they thought if they got me on it they could get something out of me? But I had nothing, I was addicted to heroin. Anyway, the worst experience I ever had with a crack addict was a good friend of mine for awhile. She didn't smoke all the time but when she did, she became awful. We were squatting in some apartment and it was just the two of us at the time. She decided to smoke one night and nothing I could do would change her mind. So she did. The entire night, I had to sit perfectly still as she constantly checked every corner of the room for the person who had snuck in. Nothing I could say to convince her there was nobody there. If I moved, she got scared and panicked, thinking that I was trying to hurt her. I was so scared, I thought she might kill me. Eventually I convinced her to let me go buy her some beer. When she drank it, she calmed down. After that, I would beg her not to smoke but she didn't listen. During the day, everything was great but then night came and she turned into a monster. Eventually, I couldn't stand being around her. It was just too much. The saddest thing was when she took me to see her daughter one day. She had lost custody because of her drug use. But seeing her daughter just made her want to get high because she felt so hopeless. It's a vicious cycle.

Update 2 - I wrote this answer before I relapsed and went to rehab once again. I just want to say that a decent rehab in 2017 will now cost you upwards of $1000/day. Good f’ing luck with that.