my experience has been completely favourable. when I caught a virus and was unable to drive hgvs for 12 months I got paid a very good pro rata rate from day 1. they also stopped premium payments for the length of claim. also I had a week off when had food poisoning, again no probs.
however, be careful, as with all insurance policies, usually there are lots of small print and exclusions to watch out for. tho my cover was from day 1, and there was no exclusions. if you were unable to work, for any health reason, you got paid. anything over 3 days and you needed a doctor to fill in form, which cost up to 20 quid, depending on your docs.