Except they recently discounted the price of their top AYCE Advanced package (uncapped data, uncapped tethering, unlimited calls and texts) down to £20 pcm on 12 month contract. I was rolling along on the 600 minutes version for two quid a month more - switched in a heartbeat.
I accept providing certain levels of data costs to operate radio networks, provide infrastructure yada yada - I'm happy to pay a reasonable cost. £20-25 for unlimited everything with no horrific traffic shaping past a low threshold (looking at you, giffgaff) is a pretty reasonable deal. The mega cheap tariffs are simply feasible because call and SMS infrastructure is there and total cost is so low (and utilisation falling year on year) the providers don't mind. Data is the cash cow but building out infrastructure capacity and sufficient backhaul is increasingly costly.
Three have had the most generous packages for a while and brought in some of the things like inclusive roaming a good couple of years before other providers. Many years ago I started off on Voda, migrating to O2, then T-Mobile (Flext with Web 'n Walk for pretty high allowances of 3G data and that was back in 2005!) which became EE, but I've never been as happy a customer as when I've been with Three. No mucking about with retention deals you have to hassle for at renewal, just simple prices, good features, a few nice perks and overall good reliable customer service. Can't speak to everyone's experience but I've been a Three customer for over a decade.
Three had coverage issues in early 2000s (remember the voice-only 2G Orange backfill areas?) but coverage and speeds have significantly improved since. MBNL partnership benefitted both providers in rural areas. Three's weak point remains indoor coverage in some areas due to spectrum allocations and cell site paucity (my employer has mobile repeaters installed in our building, but only for two telcos)...
This won't last forever though. Their service overall is great and prices are amongst the best in the industry. Even with the option of going to Voda or O2 on one of their top packages with company scheme discount, the packages can't compete. At the moment I think no other major provider can match Three on tariff features, data allowances and general levels of customer service. I was happy to recommit for another year.