For years, I've bought Wi-Fi only iPads figuring that I almost always had Wi-Fi available in the places I commonly used my iPad. Last year when I upgraded to the iPad mini with Retina Display I opted for the Wi-Fi + Cellular model to allow additional flexibility. Because I already have a "share everything" plan with Verizon, it was only an additional $10.00 per month to add my iPad to my shared 2GB data plan, seemed like a reasonable thing to do.
After living with the iPad for about a year now, I'm finding that I very rarely use the cellular data connection and wondering whether it's worth the extra cost. When T-Mobile came on as a carrier for the iPad they introduced a free 200MB per month plan which would probably suite most of my needs. I set out to see if I could move my "Verizon iPad" to a T-Mobile free data plan. As it turns out, the process was easier than I thought, assuming you have a compatible iPad. As of the writing of this post, this process seems to worth with the 2014 model iPad Air and iPad Mini with Retina Display.
Acquire a T-Mobile SIM
You will first need to acquire a T-Mobile SIM for your iPad. These SIMs normally cost $10.00 but I've found they will regularly go on sale. For example, I bought mine for $0.01 using coupon code "SIMDEAL" which I have seen featured with some regularity. I purchased the "T-Mobile Starter Kit Nano SIM" that advertises compatibility with the iPhone 5. You can order online or pick one up at a T-Mobile store. Make sure you order the Nano SIM. It should look like this:
Replace your existing SIM with your new T-Mobile SIM
Once your new SIM card arrives you'll need to replace your existing SIM. I suggest you hang on to your old SIM for a while as you may decide to switch back to your old provider. You'll want to carefully remove the T-Mobile SIM from the cardboard carrier and put it in the SIM tray of your iPad.
Once the new SIM is firmly in place, turn on Cellular Data on your iPad and restart your iPad.
Connect to the T-Mobile Network
Once you have restarted your iPad it may take a few minutes for the iPad to recognize the T-Mobile network. Make sure you're in an area with reasonable T-Mobiel coverage so you can setup your new T-Mobile account. Once your iPad registeres the T-Mobile netwok choose:
System Preferences > Cellular Data > View Account
Setup A New T-Mobile Account
If you don't already have a T-Mobile account setup with this SIM (if it's a new SIM you won't) you'll want to create a new account.
Choose Your Plan
I opted for the Free 200MB a Month Plan
Create a T-Mobile Account
You'll be prompted to enter information and create a password to create a T-Mobile Account. Note that after choosing the 200MB Free plan, I was never asked for any payment information.
A New Data Plan
You'll have to agree ot the terms and conditions and wait for your SIM card to activate but you should be done. You'll receive a confirmation email letting your know your plan is active. Your free 200MB will reset every 30 days and you can check the status of your data usage in System Preferences. If you need more data you can buy additional data a-la-carte. You should also be able to swap out with your original SIM if you keep it, but will probably need to restart your iPad each time you swap SIMs.
While I'm happy to have my free 200MB of monthly T-Mobile data for my penny investment, in practice I've found the T-Mobile network to be so spotty in my area that it's valuable is questionable. According to T-Mobile's coverage maps I have "good" network coverage at my home and "excellent" coverage at my office. Unfortunately I can't reliably send or receive data from either location. Other parts of town, the service is reasonable. Of course network coverage, speeds and reliability are going to vary wildly from location to location so your mileage may vary. That's why I suggest you keep your old data plan in place until decide if this is a plan that will work for you.