Bree, we wrestled with this when we first began using Marketo. First we created Smart Lists and then we thought "why duplicate the work?" and stopped.
Then we ran a couple of campaigns which impacted far many more leads than anticipated because of a simple and/or error. Another downside is that as our Smart Lists got more complex, it started taking longer and longer for our campaigns to execute, which threw us off for time-sensitive events.
Now what we do for emails campaigns (and I am talking ones targeted to promote specific events, such as trade shows and/or webcasts) is we use a smart campaign to populate a static list that we then use for a second out-bound email campaign.
Pro to this - you get accurate count of your outbound email flow before you begin, and your mails go out at exactly the time you want them to.
Con - 1. There are obviously extra steps to that must be taken. 2. It does leave you with a lot of "Assets" and Smart Campaigns which clutter up your Marketo instance (unless you’re diligent about taking the time to decommission everything.)