My missus uses them and is happy.
The only issue-ette she had is that there’s a “credit limit” for when you go over your allocated mins / data (on pay monthly – so not directly relevant to you). For her it was around £60. She was abroad at the time and they cut her off until she sorted it (fair enough really). Thing was to sort it she had to do PAYG top ups. The chaps on the phone were super helpful but they couldnt settle the balance – it had to be top-ups. But the most she could do on their system was £20 iirc. Anyway her credit card company got suspicious on her 2nd top up in as many minutes and the card was stopped.
Overall though – she’s still with them and I’d happily switch to them. I’m not sure if their support is also with O2 – I suspect it is but they knew their stuff anyway (UK based too).