On a normal smartlist, you're able to add filters, then click on the Leads tab and see the results of the query (along with the fields that are in whatever view you have selected). However, we cannot see the values from the fields that are "trapped" in a custom object. This is a pretty basic QA step for marketers to understand that their audience is comprised of the people they expect. As a workaround, we have to currently drill into each lead record that we see in a smartlist involving custom objects, then navigating to the Custom Object tab, then going through the picklist to identify the specific custom object you are targeting.
This isn't very scalable. So please adjust the "Custom View" feature so that we can select fields from a given custom object and display it on the "Leads" tab of a smartlist.
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