Glad I switched
I switched from Vodafone to Three about a year ago, after 10 years with Vodafone.
Don't regret it one bit. I was lucky if I got any signal at all with Vodafone, and when I did it was usually 2G. I also had a poxy 500Mb data limit.
With Three, for the same amount I was paying Vodafone, I get a signal in most places, It is always at least 3G, and usually HSDPA, 4G at no extra cost, and "all you can eat" data.
So, why did I stick with Vodafone for so long?
Well, every two years, at upgrade time, Vodafone would offer me something decent for the same tariff, or a modest increase. Last year they only offered a Windows phone (not at all interested) and some "landfill" Android devices to replace my HTC Sensation unless I either upped my tariff by something like 30%, of forked out a huge (£150) upfront charge for something I was interested in (a Note 2)
Went to Three and got the Note 2 for no upfront charge and £3 more a month. Goodbye Vodafone.