Been with o2 since they were Genie. Had a Business contract with Vodafone and switched that to o2 in 2002 after the year was up.
Never had a problem with them.
When my daughter had a Large bill they were so good and helpful and let her still use the phone and pay the big bill off in installments without damaging her credit rating (she didn't realise Cyprus wasn't part of the EU)
Windows 10 Home vs Windows 10 Pro comparison: Here are the extra features you get in the Pro version