about 20% of the JB fleet are Embraer ERJ-190s. those are the ones that have 4 seats across, grouped in 2s. I believe they all have XM radio in addition to DirectTV. I'm not sure which flights they're really used on, but I suspect it's shorter flights. The only time I've been on one was flying between LGB and LAS (incidentally, that flight didn't fare too well for DSS but the XM worked just fine).
the other 80% are Airbus A320s which are 6 seats across, grouped in 3s. and these planes all semi-recently had the first row removed, giving the seats before the exit rows the largest amount legroom. previously, JB had removed the last row to increase the legroom for the seats towards the rear of the plane. Some of the A320s have XM as well.
snacks and soft drinks are unlimited on JB as well. yours for the asking.
incidentally, make sure you book and check-in online. you get double the trueblue points, which range i think between 2-8 per domestic flight, depending on range. for example, I believe JFK-MCO is 4, but booking and checking in online gets you 8 each way, or a total of 16 per round trip. 100 points gets you 2 one way passes. Each passenger gets their own respective points.
during the holiday season last year, they were even giving bonus points of 2 each way when you checked in online (incidentally, this included using RAC at the WDW resort)