You don't need me to tell you that your kid is addicted to their phone. In fact, the ones who aren't are actually outliers. And the addiction has spread to adults as well, but it's an even bigger issue with kids because they have been raised on their phones, and don't know a life without it.
The biggest problem with phone addiction is actually spiritual. You are never forced to be alone and connect with yourself, because the "out" is always right there.
"Solitude is the process of connecting with your deepest self, which is the source of creativity, love, compassion, joy, and peace."
Deepak Chopra
Phones also degrade resilience and coping skills. Being able to instantly satisfy yourself with little effort may sound great, but it wreaks havoc on attention span and work ethic.
Phones degrade situational awareness and appreciation of your surroundings. If you are engrossed looking down at your phone, you will not notice, see, and appreciate things around you that need to be noticed, seen, and appreciated.
Phones increase your stimulation threshold. (How much it takes for something to interest you.) After years of watching short videos and playing games that you personally picked at-will from almost unlimited choices, you will not likely have the capacity to sit through a 2-hour cerebral movie, no matter how great it is. Phones offer the entirety of mankind's cumulative knowledge at your fingertips ...But kids would still rather just play candy crush, or even worse, highly inappropriate sexual or violent content. These things are just more stimulating to kids than learning about the structure of the atom or the rise and fall of the Roman Empire.
Cell phones also degrade developing responsibility, prudence, and forward thinking. It's no longer as important to make sure your plans are in order ahead of time. You can just plan as you go and adjust as needed on the fly.
Texting also can be detrimental to social development. Texting is quite handy when you don't want others around you to hear what you are saying, or you only have something short to say, but it becomes an issue if you text excessively or will only text and refuse to ever speak to anyone on the phone in person.
This is why:
Excessive or exclusively texting shows that you would rather not be responsible to have to think of what to say on the spot. Texting lets you take your time to say the exact right thing. This may sound like a good thing, but it degrades developing the skill of holding a live conversation. And not hearing the emotion in someone's voice degrades developing emotional intelligence, empathy, and social skills. How many times has someone taken a joke text seriously because they can't hear the tone of your voice.
Now let's add them up.
Decreased connection with your inner self.
Decreased conversational skills.
Decreased empathy and social skills.
Decreased resilience and coping skills.
Decreased attention span.
Decreased work ethic.
Decreased planning and organizational skills.
Decreased situational awareness.
Increased stimulation threshold.
However, all that being said...I still think that at a certain age every kid should have a phone on them at all times for safety reasons. Kidnapping, bullying, and crime rates in general have dropped drastically since the year 2000, when cell phones first became mainstream...I don't think it's a coincidence.
And giving or taking away phone time is a highly effective way of getting kids to do what you want.
"So, are they terrible or great?"
Both, depending on how they are used. The same can be said of several other things. Many drugs are either deadly or lifesaving, depending on how they're used. Cars are extremely beneficial or deadly depending on how they are used. The list goes on.
"Ok, how do I make sure my kid is using their phone the right way?"
Kids should not have their own phone until they are 7 years old. Toddlers become addicted easily and quickly.
Establish phone-free zones and phone-free times. These are up to you, but the dining table is a hard no.
Kids should NOT be on their phones during class, unless specified by the teacher or emergency.
Set parental controls. Make sure that you can see anything that they see.
Offer alternatives such as sports, skills, and hobbies. (See: Sports.) (See: Skills and Hobbies.)
Educate them about the aforementioned threats to development that phones possess.
Summary
It's easy for me to say, "Tell your kids to put their phones away.", but realistically, I myself have enjoyed the benefit of how well phones keep them occupied. And I too have caved in at times and let them have the phone just to avoid volatile pushback that is sure to come. It takes a lot of strength and discipline to enforce taking the phone away. The best thing to do is establish limited phone time right from the beginning, and prevent addiction rather than treat it after it sets in.