I switched from EE about six months ago.
Been good so far, I use a dual Sim phone with the second one my work number, that’s still on EE.
For data they’re pretty much the same, I suspect BT are piggy backing someone else for the 2G stuff as I get signal where the EE Sim is dead.
No idea on the customer services side, ordered online and the number transfer went through as expected. Even changed the data package online as I was using more calls than expected. All went through smoothly.
Only downside is the typical online voucher con all the big companies do. If you’re offered Amazon or M&S vouchers for ordering, take screen shots as proof later if needed. They claimed records showed my order didn’t qualify and wouldn’t pay out on the Amazon voucher. It was only a £20 one and I’d no proof to push it further. Don’t think that kind of problem is specific to BT as all the big companies offering vouchers seem to screw it up frequently.