Why You Should Write Your Own O/R Mapper ... Or At Least Try

Because you will not use it. But in the process you'll probably learn a lot of things about what a data access layer is. And more important, you'll be able to judge the existing O/R mappers and chose the one that fits most of your needs.

The down side is that the period of time before you choose to "upgrade" to an already existing solution will seem a waste, and all the code you've written must be thrown away. That won't be an issue if you keep in mind that things are replaceable and if you build a loosely coupled designed so components, like the data access layer, can be replaced, almost, effortless.

This pattern applies to other general "patterns", like dependency injection containers. Hands on experience with their "entrails" should give you a better understanding of how they work, and a better appreciation of the ones that are already available. And chances are you've already tried to build something similar but never knew exactly what you wanted, or what the big picture of the solution is.