I'm with Exeter Family Friendly, but I've only been with them since October 2013 and haven't made a claim yet. The reason I picked them is that their policy for pre-existing conditions is really good: most companies will only cover a pre-existing condition if you go for two years without any symptoms or treatment whilst you are with them, whereas with EFF they cover any condition once you have been with them for two years (ie whether you've had symptoms or not).
They don't do no claim bonuses, but the advantage of this is that your premiums don't change if you make a claim.
They rate well, but like I said I don't have any practical experience of dealing with them I'm afraid!