The 2nd thing that I found a bit constricting is when you have a database table that looks like this:

car_id, bigint
car_name, varchar(100)
manufacturer_id, bigint --foreign key reference to another table


and a matching class that looks like this:

public class Car
{
private int _id;
public int ID
{
get { return _id; }
set { _id = value; } //you'll see why I have a setter in a minute
}

private string _name;
public string Name
{
get { return _name; }
set { _name = value; }
}


private Manufacturer _mfg;
public Manufacturer Manufacturer
{
get { return _mfg; }
set { _mfg = value; }
}

//more properties and methods below
}

Okay, simple enough. Now the difference you see here is that the database has a single integer value for the reference to the other table (manufacturers). The class has an actual object, not just a manufacturer_id. Now picture we are on a form creating a new car, and we have a dropdown list of manufacturers (which, btw, in my NHibernate framework is as easy as:

dropdown.DataSource = Manufacturer.GetAll();

). Okay, so the user types in a name and selects a manufacturer from the list, and then clicks a button to save it:

private void btnSaveCar_Click(object sender, EventArgs e)
{
Car car = new Car();
car.Name = txtName.Text;
car.Manufacturer = //what do we put here? we have 2 choices

//we can get the entire object from the database (wast of a call)
car.Manufacturer = Manufacturer.Get( ddlManufacturers.SelectedValue );

//or we can create a dummy object
Manufacturer mfg = new Manufacturer();
mfg.ID = ddlManufacturers.SelectedValue; //fool our object by just providing the id

//finally, save the object
car.Save();
}

Now, the 2nd method I don't like at all, for 1 because there is no reason to ever SET the ID field, and 2, we are dealing with a fake object. If we make further changes to the car the updates could ripple down to the child objects (and may cause undesired results). So I have stuck with method 1, using Get and wasting a call to the database, with the advantage that I am working with a true object.

Anyway, I have yet to do any really complex querying, and that's really the part I am dreading, but other than that I have saved tons of time by not writing any SQL at all.

You can either use ISession.Load instead of ISession.Get, this will give you a proxy and won't hit the database, or you can enable the second level cache, then the object will be cached when first loaded and you won't hit the database next time.