I've done Arnhem on a number of occasions, both the whole campaign and various tactical/grand tactical actions.
Whole campaign was generally done in one sitting using operational sets (albeit taking all day) although on one occasion we had a separate operation planning day too. Tactical games were usually regiment/brigade sized covering key events in the campaign (Frost at Arnhem bridge, the 107th Panzer Bde at Veghel, Elst etc).
I've also played it a lots using board and computer games.
Along with the RF supplement mentioned above, the Command Decision/Test of Battle Market Garden scenario book is an excellent resource with lots of scenarios and photos with terrain ideas.
We didn't really make any special terrsin at all as it is all fairly standard north european scenery albeit with lots of river, canals and bridges. I've been up and down the Eindhoven-Nijmegen-Arnhem corrider and it is just like northern France or even souther Britain, albeit flat and wet (apart from the hilly bits). We did build a big version of Arnhem bridge for the Frost game out of cardboard. A bit crude but did the trick. It was mainly there for Graebners battalion to get shot to bits on, although we also had a random event with a badly repainted Leopard charging over the bridge from the south.
Force-wise, MG is one of the thsoe campaigns where you can have anythign and everything – penal troops in tropical uniforms rubbing shoulders with veteran FJ, King Tigers and Flammpanzer Char Bs and of course US & British airborne and British & Belgian infantry and armour.
Campaign-wise (as in the account above) Nijmegen is key, Arnhem is virtually a side show. The British are generally strong enough to bull into the city and even if they fail they can easily hold off the whole of IInd SS Panzer Corps and KG von Tettau on the north bank of the Rhine. But the whole campaign hinges on Nijmegen, if the Germans can slip units across the Rhine at Pannerden to reinforce the garrison and prevent the 82nd taking the bridge, the whole campaign hangs in the balance. 15th Army can really mess things up by pushing strongly for Grave rather than wasting its time around Best & Eindhoven as well.
Conversely, the 82nd need to focus on taking Nijmegen bridge rather than mucking around on the Groesbeek heights.
Anyway have fun, one of my favourite WW2 operations. Not sure of the specifics of the FGOW campaign, but it should be fairly well balanced, maybe 55:45 in favour of the Germans. We've certainly managed to win it as the Allies (and go down to horrible disaster of course).