They're not in the 'Big 6', and they have some good tariffs.
I joined a couple of years ago, as they claimed to be not necessarily THE cheapest, but to be always very competitive as other companies go up and down. So far that aspect is holding up.
However, I would advise against joining just at the moment. Things started to go badly at the beginning of the year when they adopted a new whizz-bang computer system, which promptly went into meltdown. Things got so bad the the CEO sent an email to all customers entitled "My sincere apologies". A couple of sample paragraphs -
=======================
We have migrated over 7 million pieces of data onto the new system and, like any big change, we have experienced a number of teething problems which are preventing us from giving some customers the service they deserve. We are working extremely hard to fix these problems as quickly as possible to ensure our service levels return to those which customers are entitled to expect.
The changeover has generated a significant number of customer calls and email enquiries, which has reduced our ability to respond within satisfactory timescales. This is not the level of service we want customers to experience. If you have had cause to contact us recently and our speed of response has not been as you would expect, please accept my sincere apologies.
=========================
Things were supposed to be sorted by the end of May, but it hasn't happened yet, as evidenced by the constant stream of anguished customers on Twitter. My Direct Debit of 15 July wasn't taken, despite ample funds being available!
I'd recommend them, once they get their act together.