If you have a lot to lookup or deep Quebecois roots, it's invaluable. If you only have one or two names, it's easy to get a volunteer at a major research library to do the lookup for you. This is available at most major research libraries and genealogy collections as a set of bound books.
The important thing to remember is that it centers on Quebec and just a few other French settlements in Acadia and the US. If you're looking for an Acadian lineage, there are better resources for the period prior to 1754. Also, the PRDH records only go up to 1799 at the present time. After that, you'll need other resources.