I've been with Voda for about eight years, bar a one year break when I went to Tmobile purely for the phone they were offering. While I've had a few bad experiences with Voda, they've generally been fine, whereas TMobile were uniformly bad - and their network was hopeless too.
IME Voda have the most consistent coverage - I live and work in a fairly rural area, and it's no coincidence virtually everyone I know is with Vodafone. They've never really let me down wherever I've travelled, but when friends visit with Orange, T Mobile, O2 phones they invariably start waving them around in the air or leaving them by the window etc to get a signal. All the networks have at least their share of bad customer service, Voda included, but at the end of the day it's the performance that really matters.
One thing I really dislike about all the networks right now is their constant pushing for 18 month or longer contracts. Everytime I upgrade I have to go through the same rigmarole of being told that 12 month contracts don't exist anymore, they can't possibly do it anymore, etc etc, until I eventually wear them down. All the shops and websites are the same; they use every trick in the book to get you on a longer contract, by hook or by crook.
My wife recently upgraded at Phones 4U, insisted on a 12 month contract only, they swore blind that's what it was but explained that "their system could only print 18 month contracts". They "confirmed" with Vodafone on the phone in front of her that it was 12 months, handwrote on the contract that it was 12 months, two weeks later we got a letter from Vodafone saying thanks for the 18 month upgrade. To be fair to Voda it looks like this was just Phones 4U lying to get the sale, and Voda are saying they'll fix it if we take the paperwork into a Voda shop. If they don't, that's one handset going straight back.