in st5 postcode this is by far the best network for mobile internet speeds. The pricing has gone up recently though, but for actually performance I rate them the best of all the UK networks.
I dont find many areas that suffer from coverage in general at all though.
I actually had a text from three last week telling me the st postcode actually has had an upgrade as well which they claim improves indoor signal in areas that were weak. Not that I found any weaknesses.
For example my local morrisions soon as i walked into the building on vodafone and with o2 the signal cut dead for data and calls. I assumed all networks suffered as a result of the roofing material etc. (metal) but when i switched to three, i noticed that my singal was fine in there, and in actual fact, mobile data is perfect too. Full 4g in this building.
Of course other areas may have different results.