I've been with Zen for years. I pay £47 per month for 1000 anytime inclusive minutes to the UK, including the line rental of my landline and 100Gb download of VERY reliable broadband service. Basically it never goes wrong other than my router (wireless network) sometimes needs a reboot; but that takes seconds and Zen can't be blamed for that.
Yes, it is more expensive than others, but I work from home and I tend to use the internet a lot around 6pm - 9pm at night for conference calls to New York and the like and therefore having a consistent internet connection is important. If I moved to Plusnet to save money I fear I'd just be let down by crap customer service and internet throttling just when I needed a reliable connection.
I also use SkypeOut to make international calls so the call quality is rather important to me.
I'm tempted to move to BT's Infinity or Virgin Cable once they prove themselves reliable but for now I can download YouTube or a movie in about 1/4 to 1/3 of the time it takes me to watch it so that is plenty fast enough.
I switched to Zen from BT ADSL a few years ago. The speed with BT was fine, but the reliability of the connection sometimes left a little to be desired. I read http://www.thinkbroadband.com/ (formerly adslguide.org) and the reviews for Zen internet were consistently, and I mean 100%, great for Zen Internet. So I phoned them up and got them to switch me over.
And I have to say they are amazing. I consider myself a fairly demanding customer for broadband. I work in IT and work from home a lot so speed and reliability are pretty important to me.
Not only has the service never been unavailable, it always blisteringly fast. Ive had a few queries that I had to submit to their support desk and always had a quick turnaround and a full and detailed response. No fobbing off.
They are a touch on the expensive side when compared with some of the multi-play vendors out there, but in my opinion, more than worth it. With so many buyouts and mergers these days I hope they can stay independent and continue to do what they do so well.