Well, you can use it for drop testing.

New Rates have come--NINE CENTS--USA
 

I tested my Truphone SIMs last night and the new nine cents for calling and SMS are working.
Free incoming calls.
Free incoming SMS [still]. Voicemail is free.
These tests were USA to USA.

Wow! Like some of their new voice rates... Data pricing has a lot to be desired. Always had real good luck with Truphone. But have let my SIM account lapse... Would buy another SIM if they could provide some better data pricing...

Confirmed the free incoming calls here in Germany last night. I'll definitely be using my Truphone SIM more, especially in the US. Interesting strategy they have going focusing on these select "popular" countries, a list which will hopefully grow longer. For now, Piranha still has them beat (only slightly) for the rest of Europe, and far more so globally, where Truphone still charges over $1/minute for calls (outgoing and incoming) in many countries.

Strategy?
 

I am not so sure this is a strategy, as much as a necessity in that these were the deals they were able to get and either through uncooperative local providers or not having tried hard enough yet to get other providers signed up to the plan. They have been quite open about this strategy since they started and things have just not happened. I am pretty sure it is incumbent local carriers who have blocked Truphone's progress.

Still, a good deal for some countries, I'll be lending someone a phone for a week in the US with the truphone SIM, it's generally more reliable than Piranha, and will be cheap for calling back to UK and Germany. Wish Piranha could sort out the texting issues and make it easier to change the primary number from US to UK and back.
T-Mobile US free roaming and cheap calls has become our easiest choice when traveling.

Just set the network to automatic and you are good to go with Truphone.
With Piranha, now that both AT&T and T-Mobile is the same price [0.16 Data Per MB]
the issue is where are they going to be and are they going to make phone calls.

Is the primary number even changeable? You make it sound doable but too hard.

for Piranha, they claim that if you call them, they can change the primary from US to UK, but didn't work for me.

So now that TMo USA is so good for international roaming, and I was about to archive my collection of international sims, I realize that I might need them for US travel - TMobile now has amazing coverage in almost every country in the world, thanks to its partners, but very spotty coverage in rural US, where I've been recently. And there are some large areas (eg. central NH) where the roaming agreement with ATT is not in effect.
So I've taken to using Piranha for backup in the US on ATT (which is more expensive for phone, slow 2g data, but at least a signal!)
Truphone defaults to Tmob, but I've read that you can force it to ATT via phone settings - but that doesn't work on my phone, says sim does not allow registration on that network. Wonder if part of going to cheapo 9/9/9 pricing required abandoning agreement with ATT, who apparently charges a lot more than that to MVNOs?

Well, I quick went and checked the nearest phone I have and it fired up on AT&T which I assume was the network I that it had been connected to last time I turned the phone on over a week ago. So, AT&T seems to still work. In my experience, if your phone lets you choose networks it might take 3-5 tries choosing the 'other' network so it 'takes'.

I have 11 Truphone SIMs and have had them for years and this has been true always. The big if is whether your phone allows network choice. Some of my phones do not.

Bet you never thought of international SIMs for US roaming purposes!