dashes - Comma vs. em-dash

When is it appropriate to use em dashes instead of commas? I have tried commas in this sentence, but it seems too disjointed:

I'm going to be here at Zep, just up the road from KO near Howell Mill, for a few more weeks, and thought you and Kelly might like to meet somewhere for lunch one day.

Answer

In this instance, em dashes are more appropriate. Em dashes are commonly used as alternatives to parentheses and the location of Zep is definitely parenthetical to the rest of this sentence.

Thus, suitable alternatives might be:

I'm going to be here at Zep (just up the road from KO near Howell Mill) for a few more weeks, and thought you and Kelly might like to meet somewhere for lunch one day.

or

I'm going to be here at Zep—just up the road from KO near Howell Mill—for a few more weeks, and thought you and Kelly might like to meet somewhere for lunch one day.

Personally, I think the em dashes are overkill here; they highlight the parenthetical clause in a way that I don't think is truly necessary, but it's up to you.