I have used them since they first had roaming sims, may be about 3 years ago. Was using their original sim only for international travel, since the US rates were untouchable. I did not have any problems. They were down for close to a year or so, while they revamped their offering, and came out with another sim almost a year ago. The new offering has very good US rates that rivals most US prepaids, so it has been used extensively in the US by my son. In general, it has worked well.

Edit: All of these international sims have the occasional and sometimes long-term issue. For example, telna does not have VM. And in my experience, when roaming internationally, you may get the "sim card registration failed" message and you have to restart the phone 2 -3 times. Just happened to me in Nigeria a few weeks ago. Thanks to DrNewcomb for pointing that out.

A while back Telna came here with a free SIM offer for folks who would send them a SIM from a defunct service. I took the bait and have been reasonably happy. There are occasional issues with failures to register. These are normally resolved by recycling the phone's power. Initially I had a problem receiving texts from CSpire (a US regional CDMA carrier) but that has been resolved. I have not traveled internationally since I retired but have used the SIM in the US. I sent it abroad with family but they never used it except in the US.