I have taken several flights w/ Midway, my first happened to be a flight from FLL to LAX(no longer a city served by JI) via RDU and I was very happy by their service. I recently took a trip on them(one-way) from EWR to FLL same-plane via RDU, and again, I was delighted.
You have chosen to fly JI from BOS to MSY via RDU. On the BOS-RDU leg, I would expect F100s and newly delivered 73G's on the route. JI's concourse at RDU has a linear pattern(meaning all gates are lined up in a row), so if you deplane at Gate C20 and the RDU-MSY flight boards at Gate C3, I would anticipate a 5-10 minute walk. It's not too bad, unless you really hate walking.
Their service is good for such a low fare. All flights on the BOS-RDU and RDU-MSY have a cold snack service.
Because JI has frequent service from BOS to RDU, I don't think that you're flight is going to be full, however, expect a full house on the RDU-MSY leg. You'll be on a CRJ(50 seats) unless a 73G is added which might happen due to the load factor.