I have had the mattress for just over 15 months and it has developed what appears to be a split in the top of the mattress on one side.  I have complied with all of the warranty conditions and under the terms of the warranty, I am entitled to a replacement mattress, and will pay a contribution to the transport costs. 
With reference to the star rating, I have given a five star rating, as the mattress has been very good - apart from the the split which has developed.