I've bene using them for close on 2 years. The first year or so of that on a standard plan when I was in the US. And for the past ~9 months on their (no longer offered) $5 light plan which I use mainly from Australia via Wifi/Backup calling.

All up I can't fault them. I haven't used any of their more advanced features like jumping between networks - partially because I don't have a need, but mainly because those features didn't exist when I was still in the US. The only problem I've had is that Pixel Backup Calling (wifi calling over a 2nd data SIM) didn't work on their 'Warp' (Verizon) product, but I ported over to their T-Mobile product and it works fine. That was 18+ months ago, so may well have changed by now.

The do change their plans (and prices) fairly frequently, but I think thats just a side effect of them growing and trying to find the right price/service points. The plan I'm on is no longer offered and hasn't been for around a year, but I'm grandfathered into it, so I'm still only paying $5/month (only catch is they recently removed the ability to buy additional data on this specific grandfathered plan - which is annoying as otherwise I'd just do that whenever I was in the US).

Their CEO is active on Reddit, and generally the feedback there is very positive, both for the product and the support.

They do have referals that give free service, but I don't think Flyertalk allows posting them?