I’ve found they don’t offer as good value for money as some of the options already available in the UK. There seems to be a huge markup on the amount they charge on data. Might be a good option if you’re needing a ton of data but I’d say it would probably be better to be using EE’s roam abroad pass at £10 if you’re with EE or just buying a SIM card from a provider that already allows roaming (Such as O2, Tesco, or Three PAYG) to roam in the EU. If you’re roaming in the USA and you can live with 12GB for two weeks just do that.
These roaming esim providers might be helpful if you’re needing a ton of data and in the USA. However I’ve been finding roaming is with a UK SIM card has overall been better value for money on my part. Everyone will obviously have different wants and needs.