I've just joined (reluctantly - ex-NHS nurse), and think it's important that people read the small print (I did, fortunately), as it really isn't an alternative to the NHS, but rather offers some additional services (hence it being so much cheaper than the big names).
What specifically appealed to me was the easy access to a GP (24 hours apparently) for my daughter and son-in-law (I've done a family membership), as their London GP surgery is completely swamped, and often all they really need is someone medical to tell them whether a particular symptom needs immediate attention or can wait.
Note that, other than the GP support, nothing much else can be accessed until you've been a member for 6 months. However after that there are various minor procedures, plus physio and psych support that are covered. It doesn't cover big orthopaedic requirements, or cancer, or cardiac. So, I'm looking at it more as an 'always available' GP than anything much else.
(My other thinking is that, if the NHS does go down the pan - sob! - and we all need private cover, Benenden might look more kindly - in terms of cost - on existing customers than new ones. However, I have no inside information and could be entirely wrong!)
Hope that helps :-)