We have all heard the reasons for using an orm. What are your reasons for NOT using an ORM?
I am an ORM believer, but as a consultant I find that ORM usage is actually very low -- which is strange to me. Many shops refuse to use them, and actively tell their employees not to use them (I guess their stored proc are all they need).
I use ORMs for DB transactions that will have little overhead anyway; simple, atomic CRUD tasks that don't occur multiple times a second for example. However, when you have a large query that requires many joins and parameters and will return a potentially large dataset, you really need to get down to the SQL level to fine tune it to get the exact data you need and to make sure the query is optimized.
There is pretty good reason to not use one if the application is dealing with lots of bulk importing and exporting of data. This functionality is now being provided by some ORMs and it is improving but it is still measurably slower than doing it manually.
Not using one will probably decrease developer productivity but if the solution requires the best possible bulk data performance, that is one of the costs.