One statement that I have heard incessantly is that foreach over an array is slower than for (int i = 0; i < array.Length; i++).

Finally, some good people have debunked this myth:

• Kevin Ransom – To foreach or not to foreach, that is the question.
• Eric Gunnerson – Efficiency of iteration over arrays

I personally find it SO much nicer to look at code like:

foreach (int i in array)
{
    Console.WriteLine(i.ToString());
}

as opposed to:

for (int i = 0; i < foo.Length; i++)
{
    int item = foo[index];
    Console.WriteLine(item.ToString());
}

mostly because the first code chunk is a lot more compact, and I think more clearly states its purpose.