I've been with them a few years. As you say the premiums are good. However, i must admit i have had a couple of horror issues with them, and were it not for the premiums i would have gone.
I was set up big style when i changed my car from Saxo VTR to 206GTi. I asked for a specific quote, told them when i was planning on changing the car and when i actually bought the car and rang them, suddenly the difference in premium had shot up.
I have also had to make a couple of claims - and i have to be honest, Flux didn't want to know. They just gave me the claims number for the actual Insurance company - which was a call centre in India. Thinking about it, the first time was really bad, 2nd time wasn't so bad.
Don't get me wrong, both claims were settled ok, and i was sent to reputable repair companies - it just seemed like a hastle, especially the first time.
On the plus side though, i have taken my car over to Europe a few times, and every time i've just called them and everything is fine, covered on my policy.