I've been with BT mobile for a few months and I have had no problems either. You are speed capped at 30Mbit unless you pay £4 per month extra for 60Mbit, so it's not an MVNO for speed freaks, but 30Mbit is plenty fast for everything. Otherwise, no problems with speed/coverage for me, it's identical to EE proper
IMO I think the mobile app (for checking remaining usage) is the best out of any network, very clean, crisp design. At least it is on Android.
I have also found the BT wifi access quite useful too, though if you have BT broadband (I don't) you already get that.
If you get the Amazon/Apple giftcard they're pretty quick at sending those out, though you do have to wait for the initial 14 day cancellation period to end.
I'm waiting for them to announce VoLTE and wifi calling support - hopefully it'll line up with EE's offering