Whereabouts are you getting higher speeds than EE/Three? I'm really interested as I've got a personal interest in network coverage, and particularly data speeds. Indeed, I provide a lot of data from a range of networks to RootMetrics from multiple devices I have.
O2 usually manages data speeds of 0.5-3Mbps on 3G, compared to 20-25 Mbps on 3G with EE and Three, and 20-80Mbps on EE 4G and 20-40Mbps on Three 4G.
O2 4G, which includes a backhaul update, means better 2G and 3G but coverage is still quite dire. That said, Hatfield and St Albans are done, along with Hemel Hempstead where I've been a fair bit recently. It's where 4G doesn't exist that O2 has all the problems, due to years of failing to invest and narrowly avoiding a huge fine from Ofcom. Even now, it has let its 4G roll out plans slip by about a year.
4G is however FAR better than 3G as the network technology is totally different. 3G employs something called cell breathing, and it means sites can grow and shrink to reduce interference and congestion issues, but that has obvious side effects. And the way things work, you can have poor data (with huge latency) even with 3 bars on 3G, but get speeds of 10Mbps on 4G with the phone showing 0 bars (yes, phones can show 0 bars but not 'emergency calls only' or 'no service).
Latency causes the biggest problem for mobile users, making even a fast connection appear slow given the use of apps and mobile web apps. 4G makes things super nippy in comparison.
I'd be quite happy to see the back of 3G if all of the existing sites could be switched over to 4G. Over the years, that's exactly what I expect to happen with a lot of 3G spectrum refarmed for LTE/4G use.