billhexham, I'm a bit spoiled since the MoF is in my backyard - I've visited it countless times (I'm a member, and I've contributed to a few of its exhibits). Heck, we have four different aviation museums at Paine Field - just a few miles from my house (one of which I haven't even bothered to visit). I'd rank MoF fourth among aviation museums in the USA.
First on the list would be the "National Museum of the US Air Force" at Wright-Patterson AFB (Dayton, Ohio). Second and third on the list would be the Smithsonian museums - it being close to a toss-up between the original "Air and Space" museum in Washington DC, and the Udvar-Hazy annex near Dulles airport.
About five years ago, circumstances combined to allow me visit Wright-Patterson and both the Smithsonian museums - in three different trips - over a ~3 month period. The presentation aspect at the Smithsonian is definitely better (at Wright-Patt, you need to sign up for bus ride to a remote hangar to see some of their best aircraft - including one of my all time favorites, the only surviving XB-70). But IMHO, Wright-Patterson has the absolute best collection of vintage aircraft, anywhere.
So, you're telling me Duxford is pretty special and deserving of a dedicated visit next time I'm in the UK
... Anything in particular I should know before I get there
