I've worked with a number of different ORMs, including two on the project I'm on right now, one custom one, and a handful of open-source ones on Ruby and Python

I've worked with a number of different ORMs, including two on the project I'm on right now, one custom one, and a handful of open-source ones on Ruby and Python and the like, and every single time, the story's the same. At the start of the project, when queries are few and datasets are small, all of the above worked great. But the further you get into the project, and the more complex things you are trying to do with larger and larger datasets, the ORM ends up blowing up in your face. For example, LINQ to SQL--which I want to emphasize I'm picking on only because I'm using it righ tnow--happens to silently and transparently convert from IQueryable (meaning that your LINQ will execute in the DB) to IEnumerable (which means you'll suck in all data required for filtering past that point), which means that you never have a good idea whether your query is going to be optimized or not unless you check the SQL being generated. When you have to do this a few dozen times, you start wishing you'd just written SQL in the first case. The situation gets worse when you start getting into join-heavy queries. I've never seen an ORM that actually can cover everything you need to make an app performant. It doesn't handle full joins, or compound indices, or distinguishing between clustered and nonclustered indices, or calculated columns, or some other thing that, if you were in raw SQL, would easily solve your problem, so at that point, you start introducing raw SQL. The older your app gets, the more instances of this you get, until you'd have been better off simply using SQL the entire time anyway. I love the idea of an ORM in theory, but I'm pretty solidly convinced these days that if you want objects, you should us an OODBMS, and otherwise, you should just embrace the relational model for what it is, and not pretend you're working with objects. For most people, that'll mean writing SQL and getting back query sets.