Maison Carree was included in the price of our ticket to the amphitheater (highly recommend and is…read morealready well-reviewed), and it's only a few minutes away, so we went to check it out.
The building is beautiful (based on the temple of Apollo if we remember correctly) and has been restored well. We didn't know anything about it but we discovered the whole visit is only a 20-25 minute movie about the history of Nimes. The film is pretty B-level, almost amusing at times in its emotiveness, but does provide you with a nice history of the city.
So: if you want to see a nice building, then don't pay to go in. If you want to see a bunch of Roman artifacts, then there's a separate museum for that (which we had decided to skip). If you're into cheesy historical reenactment films, then this is the place for you.