I would say taking out insurance is dependent on the boiler and heating system you have. Some boilers are very reliable and if the system has been installed correctly insurance can cost far more than paying out yourself to have it repaired.
Some boilers eg Puma, Ideal Response, older Saunier Duvalls may warrant taking out insurance.
I advise most of my customers to set up a direct debit and put the money in their "boiler repair" account.
Some insurance companies start off on a lower rate increasing it the following years. Obvioulsy any pre-exstiing faults are not covered (and there is always the "powerflush" scam to be weary of).
Remember to check the small print. Damage from scale/sludge is not covered - that can mean a whole host of parts won't be under warranty. And you won't get a new boiler unless there are exceptional circumstances and the existing one's fairly new.
Another problem is the insurance companies sub-out the work, as a result there's often little money left for quality enigineers - the reason why I don't do insurance work.
I recently had a call from a lady in her mid 80's - she had paid the big one for the last 10 years (over £2000). The Apollo boiler had only had a thermocouple in that time. The kettling heat exchanger wasn't covered (due to scale) and they said the only option was a new boiler etc - cost nearly £3000.
Needless to say she asked me to swap out the heat exchanger, £280 +fitting. The boiler will outlive her. This scenerio happens all the time.
With some customers boilers/heating systems insurance companies will never make a profit, they take them on hoping one day they can upgrade the system with a replacement or a powerflush - then they make a killing.