Well if you can only afford a 5% deposit they are a good idea, but you should always put down the biggest deposit you can afford to get a better rate and be better protected against house prices falling, changes in financial situation etc.
When house prices fall you may not think it is a good idea. Nor when rates rise and you can't afford the interest payment any more. Personally I'd not recommend having such a low stake in a property. I'd suggest patience and opting in when you can put in 2 or preferably 3 times that percentage.