Hi,
I have only bought one or two certificates so far, and it did not show any new info that was not shown in the census and IGI, and parish records.
My advice is to collect as much info as possible about a person / family in the first instance before wasting money on these. I guess it depends on what resources you have available, and your proximity to the Record Ofiice, too.
My granfathers details were different for every place that I checked, including his birth and death certificates, and the only place that had he correct detail was the 1891 census. althogh this is not always correct.
So, it is a matter of comparing all the resources available until the right information becomes available, and in this context, the certificates are not always right.
Best
Kevan