After 3 months of randomly trying Mint Mobile service, I decided to switch from T-mobile to Mint Mobile! By switching to Mint Mobile, I'll reduce my cell phone cost by 60%, but I'll loose data service quality and customer support availability. When traveling internationally, I'll need to pay extra for voice/sms/data roaming that was included in T-mobile (however, I don't use it that much and cost is reasonable). At this time, this is a good trade-off in my situation. If it doesn't work out, I'll write another post in the future with my findings.
My random observation during the last ~3 months is that voice quality is about the same for both services, and the data transfer rate is pretty good for Mint Mobile, but still not as good as T-Mobile. Actually, a few times, Speedtest showed higher speeds for Mint Mobile (!?). In areas with weak T-mobile signal, I see that Mint Mobile has a harder time to connect than the T-mobile phone does. Even when the 4G symbol is shown on the screen, Mint Mobile may not be able to connect to the Internet on weak signal areas. Sometimes the workaround was to restart the phone, but it isn't something desired.
One of the tests I performed was driving at high speed (no more than the legal limit!) on the freeway and making phone calls or with YouTube videos running. In both cases, Mint Mobile quality was good.
In regards to customer support, it is important to note that Mint Mobile's service is pretty limited. I tried to contact customer support with the chat bot, but no human replied. The bot said it would direct my question to customer support, but nobody answered after a couple of days. I also called Mint Mobile support to their phone number, but they close in the evening. T-mobile customer support is open 24/7. We can send email to Mint Mobile support, however in case of troubles, expect to be kind of on your own.
Switching from T-mobile to Mint Mobile, was actually really easy (considering all the things involved in transferring a phone number). My new Mint mobile service was working immediately, it probably took 15 minutes to receive calls, but I was able to send calls as soon as the new SIM was enabled. However, I don't know if the process is as smooth when switching from non T-Mobile providers (Mint Mobile runs on T-Mobile network).
In order to transfer a phone number from T-Mobile, I needed to provide the account number (found in T-Mobile's web site) and the secret PIN. Something IMPORTANT to consider: as soon as the phone is transferred, I lost the access to T-Mobiles web site. Therefore, get any information, PDF of bills, etc. before transferring the service!
In order to switch to Mint Mobile, I purchased a new 3 month service online, and after 2 days I got the SIM card. Then activated it online in mintmobile.com (services purchased online must be activated on the web site, and it's a pretty simple process with step by step instructions).
Since you took the time to read my post, and hopefully upvote my post, here's a bonus tip. Mint Mobile is a vendor registered in http://aadvantageeshopping.com/. This means that by purchasing through this web site we can get Aadvantage miles. As of this week, they give 1000 miles per Mint Mobile plan purchase. Also, look for the promotional code that gives free shipping for the SIM card (the code must be entered before submitting the order in Mint Mobile's web site).