Do you use field and list item validation (in 2010)

class CreateShoppingListHandler : SPItemEventReceiver
{
    public override void ItemAdding(SPItemEventProperties properties)
    {
        float price = 0;
        float cost = 0;

        if(float.TryParse(properties.ListItem.Fields["Price"].ToString(), out price) && float.TryParse(properties.ListItem.Fields["Cost"].ToString(), out cost))
        {
            if(price < cost)
            {
                properties.ErrorMessage = "The cost must not be less than the price";
                properties.Cancel = true;
            }
        }
    }
}

Bad example: using custom code – creating a custom event receiver on the item (the item adding event or item updating event)