Snidely, if a US user is traveling to one of those countries not included on T-Mobile's plan. Personally, my use/need of international SIM cards has diminished significantly.

"Happy Camp" who posted before you only uses less than 100 min./mo. when at home! Says he has a 100 min. prepaid plan from Walmart.
He is right that the first line on a TM account (which gives you unlimited in/out voice, text and data (altho data is slowed after 1G) costs $50, 2nd line only costs $30 and each line after that $10. (all come with the unlimited voice, data text that the lst line has).
As I recall, you travel a lot to Africa - and many countries on that continent are not included in the 115+ that are by TM.

I do have the 100 minute T-Mobile "Wal-mart" plan. I use Skype's $2.50/month unlimited calling plan for most of my outgoing calling. I also have my incoming calls go to my Google Voice number and if at home answer the phone via my Obihai. Google was supposed to turn off that service at some point. When that happens I'll transition to using Localphone.com $1/month phone number and have it forward the call to the Obihai.

13 days into my 30 day plan I haves used 34/100 minutes. I mainly care about data usage. But even that I haven't used that much. 220 MB, as I use Wifi while at home and at work.

Unfortunately for Android the T-Mobile "Wifi calling" appears to require a T-Mobile Android phone. I have a Google Play Store Nexus 5, which is unsupported.

Some people believe in prepaid only.
 

Some people believe in prepaid only.

Let me take a stab at why a Yank would use a roaming SIM. My wife and I are grandfathered on ATT's international iPhone plan. I get unlimited (non-throttled) data worldwide on my iPhone. This includes high priced roams. I pay US$65 a month for it rather than $35 for the privilege and there are years that it may not be a good deal, but other years where it is worth its weight in gold.

This doesn't give me cheap inbound calls abroad so a roaming SIM makes a great deal of sense. Many times I can make a respectable VOIP Over 3g call, but this is not 100% of those times. When that happens, I trigger a callback (to the roaming SIM).

Neither me or anyone else gets legal tethering on this plan or on TMobile. I have an old jailbroken phone which I can cheat with or I suppose I good switch to Android where rooting is a much more consistent experience than jailbreaking. This means that there is some times that I need to get something else online beyond my phone. For example, I live fifteen miles (24k) from the Canadian border right off the motorway connecting to the border. This month, my wife will be working three of four weeks in Canada. I was there twice last week, almost went last night, and will be there for a week this month. For these reasons, we have a contract flex data account on a hotspot for Canada. If we don't use it, it is $5 a month. We scale upto 5gigs a month for $35. (We also pay ATT $20 more a month on our family plan to extend our family plan for voice into Canada).

I have a Toggle Mobile SIM card for voice off US shores and also a Piranha Mobile card. My best friend lives in Australia and it means we have a local number when we visit. My wife's company's European headquarters in the Netherlands. Same story. My wife has a client in Hong Kong and I have relatives in the UK. My wife also transits through the Netherlands on her way to Dubai.

When we make those trips, we carry a small Nokia pentaband Asha. On none of those trips do we bother with a prepaid SIM anymore.

also, if I'm in Europe, it's useful for local friends, family, etc. to have local number, or at least, UK number to call back to, as calling to US number is expensive for them.
So I'm back to taking 2 phones, the TMobile one for free roaming and cheap calls, and a Tru or Piranha sim.

You might check out the T-Mobile LG Optimus F6 which has been selling for $100 or less in the past couple months though Newegg and Target. Do a search on slickdeals.net forums. Easily unlocked for less than $2. The thing with T-Mobile's Wi-Fi calling though is that it counts toward your plan minutes (or prepaid) which I personally find rather stupid. You can accomplish the same thing by setting up a VOIP account with the "Internet calling" feature of your Nexus 5, or using a VOIP app, and perhaps combining their use with Google Voice.

Just heard back from Truphone regarding an inquiry I made about the upcoming rate change and whether there might be a change in support for AT&T's 3G:

"Once we apply the rate changes in July AT&T will remain as our secondary network in the US so you will only be able to receive 2G/EDGE data." -- Truphone CS

So that's one area where Piranha will still have them beat.

A blast from the past.
 

On ebay right now someone is selling ['More than 10 available'] Truphone Local Anywhere No Contract Samsung E2210 Cell Phone for $99.99. These include a Tru SIM.
Two or more years ago these were selling for less than $20 on eBay and another auction site.

Yes - And thanks to you (or someone here) bought that $20 deal! Now that I can roam for free on TMobile- that phone is useless!