Anyone used an online unlock service?

Hi there, I am looking to unlock my pre-owned iphone to use a local sim soon and wondered if anyone here has used an online unlock service such as iphone approved unlock or another similar company.  Can they really unlock within 24 hours for around £14.99? They say they don't affect your Apple warranty, which has me interested. Has anyone had an experience with these types of unlock company and do they  recommend anyone? Thanks

Hi, here's my experience:
I once tried a "SIM Unlock" for an iPhone locked to T-Mobile USA network, this was a customer's iPhone SE model so I looked for the lowest price around, did find an eBay seller with the best deal at about $70 USD at the time, the seller promised a factory unlock with no risks of re-lock even after updating iOS. iPhone worked good for a few months, but starting the 4th month customer decided to sell iPhone so he did an iPhone full reset, problem was that once reset the iPhone became locked to the original network, fortunately I was able to recover payment through PayPal and issued customer a full refund.
Problem with iDevices is that most operations require server intervention, so when you reset or update iOS, it will connect to Apple servers, risking re-lock of iDevice.

Wow thanks for your input. That is concerning! The unlock companies I looked at say there is no risk of the phone being re-locked because they whitelist them in the Apple database and this should work even after firmware updates. I will certainly look to get further confirmation before proceeding with this idea. It's good news you managed to get a refund at least!