you probably know this already, but pro, energy efficient, therefore save on your bill and "green" because uses less fuel. con VERY expensive, therefore takes a good while to realise any saving. things may be different now, but when we looked into it about 4 to 5 yrs ago, ther was nothing in it for durability and breakdown costs
We've just stayed with friends for the weekend and they have a condensing boiler. It is energy saving because you only heat the water you use BUT running a bath is a nightmare!! It takes ages and I'm sure that you don't really get as hot a bath as you would like because by the time the bath is as full as you need it - its going cold! Perhaps a shower is better - I don't know......