Since you have the user guide and docs.marketo.com, that should get you started. That should answer your question.
What's your real question?
I've used the Event Check In App at 2 different organizations, one for company-sponsored live events and one for trade shows. It really depends on your use case as to whether or not it will work well. I'd say the biggest limitation is that you can only map "Check in" to one status in a channel. It works pretty well for a live event where your status is typically just check someone in and they're attended, but it can be tricky at trade shows where you may want to check someone in for visiting the booth, or attending a cocktail party or reception.
Was it worth the spend or do you think there are easier ways to accomplish that?
Was looking into trying the app. I moved us off Cvent and wanted to test if it's a comparable user experience for our events team.