You have 3 Carriers to choose from :
1) T-Mobile : Which you have and dislike
2 ) Verizon : Your coworker has Straight Talk (Verizon ) and has issues
3) AT&T : ????
I would still ask more people in your area what Carrier they have and if there happy with it .
It's about 3.5 to 4 carriers as Dish Network / Boost is building out as well.
Just moved oilBabe from AT&T to Boost Infinite. Purchased a new iPhone 15 Pro Max from Apple and signed up for Boost Infinite's "Smart Network" Her e-SIM ICCID starts with 89105 meaning it's the "Rainbow SIM" as it's called.
Boost Infinite was using either AT&T (ICCID beginning with 89014) or TMO (ICCID beginning with 89012) until they started building the Dish Network owned towers.
The "Smart Network" is supposed to be able to use Dish, TMO or ATT networks based on what you have in the area.
While they are pushing folks to sign up for their "Get a new iPhone every year plan for $60/month" you can BYOD your device and get their $25/month unlimited plan.
That is what I did for oilBabe as I just want to own the device, unlocked, and have the flexibility to move to other carriers should I need to.
The "captive" MVNOs are (or will be, Mint hasn't been acquired yet) are:
ATT - Cricket
TMO - Mint
VZN - Visible
With Dish having Boost Mobile as their pre-paid option and Boost Infinite their post paid like ATT, TMO or VZN.
And yes, as others have noted, data can be deprioritized for the MVNO customers. For our usage, it's not needed.
Since we do a bit of traveling, it's nice having access to multiple carriers.
So I have oilBabe's phone loaded up with Boost Infinite AND Visible (both $25/month) while my personal phone is on Mint ($20/month plan) and Visible's $25 / month plan.
I still have ATT on my work phone, so there is that.
I think it's a great time to have all of these options. If you don't need all the extras the big 3+ carriers are offering, one or more of the MVNOs may be a good fit.
Edited to add that Boost Infinite is only supporting their "Smart Network" on iPhone 15 right now. If you don't have an iPhone 15 (either BYOD or buy one from them) you'll get an ATT or TMO SIM based on the "home address" of the device.