I've used them twice.
First time was excellent - added modifications like there was no tomorrow and they were
very reasonable on costs for them (most they didn't add a policy increase).
Second time was a total disaster - they were completely useless to deal with and gave the impression that once they had your money they didn't care! In the end I had to cancel and due to their cancellation fees I lost £120 of my £460 payment, 1 month and 2 days in.
From people I've spoken to in the past (mainly with Jap cars); if you don't need to speak to them or make any claims then they are great and usually quite cheap. The second you actually need them then they fall apart, don't want to know and never return your calls.
The phrase "you get what you pay for" springs to mind
Of course everyone gets treated differently and service is not always the same - but beware.. There was a 7 page thread listing problems with them on another forum I used to visit.