Downsides;
There are a lot of different covers on the market and some include excess, others can include pre-conditions such as having your system flushed before the contract can begin. Many boilers over 7 years old may not be covered, as may some cheaper boiler brands.
Once in contract it is not unusual for customers to be told that the boiler they have already covered needs to be changed as parts are no longer available. I have also known of a company telling a client that the boiler they had covered (for six years before being called out) was unrepairable, the company did give a refund of over two hundred pounds though.
One of the main complaints I get is the up selling around the contracts, also when people are told they need a new boiler in order to get cover, they are not reminded new boilers can have warranty's of up to ten years. If your being told you need a new boiler in order to get cover remember if you have a new boiler you do not actually need cover your boiler is under warranty possible for many years.
Often problems can be kicked down the line as the contractor has little incentive into doing a bigger repair, repeated breakdowns are common, often ending in advice for a new boiler.
Upsides;
Should you boiler be getting on, on going repair cover may make sense especially if do not want to replace the boiler any time soon. It depends on how many repairs you expect to have, but unfortunately not being able to see into the future you just have to weigh up the odds.
If you simply do not want to worry about worst cases then cover may be for you.
Rentals, takes the management of maintenance away as the tenant can call the company providing cover directly.
Alternatives;
Line up a heating contractor for possible future repairs
If your boiler is relatively new check it's not covered, maybe also find a reliable independent heating engineer before you have any breakdowns and shop around for rates before your boiler breaks down, check for coverage too 24 hour, emergency?
Check what cover the boiler manufacturer provides to clients out of warranty.
Often the best people to look after your possible ageing boiler are the people who manufacture it typically their engineers only work on those boilers consequently they carry or have quick access to all the parts for those boilers. Many manufactures also have there own boiler cover, on premium brands this can be expensive but the service very good. The manufacturer also has an incentive to get the boiler to last - they built it.
Get a new boiler
Especially if you want cover and cannot get it for your vintage boiler, on going repairs may not be cost effective. Plenty of manufacturers now offer 5 -7 year guarantees including Worcester and Vaillant, two of the most highly rated brands for reliability. Plus you may be able to factor in some saving on energy use as you switch to a modern high efficiency boiler.
Further Reading;
Highly recommend 18 pages of opinion and experiences on a Money Saving Expert Forum
http://forums.moneysavingexpert.com/showthread.php?p=6766426#post6766426
Expert money sense opinion;
http://www.moneysavingexpert.com/utilities/boiler-insurance
http://www.telegraph.co.uk/finance/personalfinance/insurance/buildingsandcontent/9071428/Is-boiler-insurance-worth-the-money.html
I recently answered this question about boiler cover for a client;
Hi Jeff, just got a letter from (boiler manufacturer saying my warranty ran out on 19 Jan. They are offering Boiler Breakdown Cover by (Large Company) for £119.88 for a year. This is really expensive and I wanted your advice on whether I would be ok without any kind of boiler breakdown cover (what do you think ?) or if you know where I can get it for less ?
You can get it for less on your buildings or contents insurance or through an insurance company.
Obviously they make a profit so your paying on average more than if you just paid for repairs as and when.
It's very unlikely it will have problems being two years old.
Insurance is reverse gambling, so the odds are you won't have a problem but will pay more if you do.
If you go 5 years without a problem that's £600, average repair £100 - £300. Plus if don't get the boiler serviced every year you may not be covered, you may not be covered if they classify the boiler as un-repairable (I've know this to happen).
It's in my interest people don't get insurance so they can hire me to do repairs, but I genuinely think it is not worth it see link; http://www.moneysavingexpert.com/utilities/boiler-insurance
Consider self-insurance
If you want to pay out as little as possible for cover, the best option is to self-insure. Instead of paying for a policy each month, put the same amount into a Top Savings Account to build up your own emergency fund. If you have a problem, the cash is there to pay for it; if you don't, the cash is yours. The ideal - if unlikely - scenario is that you self-insure until the day before your boiler breaks down, then buy cover just in time. Yet if you're not Nostradamus, you can play the odds by self-insuring during the early years of your boiler's life - when it's least likely to go kaput - then after three years or so, bite the bullet and take out insurance.