I just did that, and it is worth it to me especially for someone like me really into commercial planes.

Took the bus at stop #30 at T1 arrival level all the way to the right when you are out of the terminal. I took the 11:35am bus (it costs 200yen each way), takes about 20 min, the bus stop right outside the museum. It is 500 yen to go in (you can get a discount voucher 100 yen off at museum shop at NRT T1 obseravation deck level) Though the open area with a few planes including a retired YS-11 is free of charge.

The museum is a little rundown unlike museum of flight in seattle or air & space museum in dc. First and second floor have indoor exhibits including a large scale 747-400 N401PW hanging from the ceiling. 3rd floor has a door lead to outside observation deck facing the cargo hold and runway 16R/34L. 4th floor has a cafe, 5th floor has an indoor 360 degrees observation deck, it also has a screen showing what time are the flights arriving. Due to I want to spend more time in NH F lounge (I was taking 5:15pm TG 77W in F), I took the bus back at 1:10pm also leaving right outside the terminal. Got back and wait for 15min or so that TG checkin counter open to checkin for my flight.

If I have more time, they also have a cut out nose section including the front wheel of an ex NW (SQ) 747-212 sitting next to the museum. It only open at certain times and I believe there is guided tour that require a fee to go up to see the cockpit and somthing else. It is their new exhibit, and it painted like N7470 (no.1 747 built) white color with red stripes and all the airlines stickers that ordered 747 at that time.