can't say I've been impressed, but I have been helped
I only have to call CS once every 2 years when I get some type of wacky tech issue. I'm a geeky kind of guy, so I know my way around technology, so if I call, it's not because I didn't read the manual or don't know how to use the phone, it's because there's an actual problem.
Back in the day, T-mo would send me a text message letting me know I had new voicemail. It was free, and came from my own phone number (at least that's how it was tagged) well, one day, I stopped getting text messages from my own number and started getting calls. If I answered the call, there was nobody on the other line, it would just disconnect. It seemed like whenever someone left me voicemail, the system would call my phone every 2-4 minutes until I called my voicemail and listened to the message. I would be at work, check my phone in my bag on break and see that I had 52 missed calls!! One legitimate call from a friend or family member, and the other 51 calls were my own number. It got where I would turn my phone off because even on vibrate it was wearing my battery down. The online/email tech support people were convinced I just didn't know how to use my phone, eventually it just fixed itself and I started getting notifications instead of texts or calls.
Last year, I had another problem with voicemail. No one could leave any, and I couldn't check. Again, CS was convinced that the problem was me not being able to use my phone. The real issue is that they had changed the VM number, and my phone never got updated with the new number. There was an update sent to all the phones six months ago, but I never got it. So when they stopped using the old number alltogether, I stopped getting voicemail. I had to plead with the first level tech to escalate, I had to swear that I am not an idiot, and convince her to pass me on to someone else. She was just flipping through the manual for my phone and even told me craziness like "oh, well, you're supposed to restart your phone at least 3 times a day every day."
I did get help, in both cases, but my only complaint is that they don't let their techs escalate to higher tech people soon enough. Those first level responders are supposed to weed out the folks who just can't operate the phone, but it's kind of insulting for those of us who have a real issue that needs to be resolved on their end. I'm still happy, because my issues got resolved, and the folks were nice.
No billing issues, no real mistakes on their end, just wacky tech issues, and only a few over the last 8 years with T-mo