Test them - one at a time.
Some work perfectly, some do not.
Aircraft models can have issues, aircraft textures can have issues, aircraft panels are very susceptible to problems. But most can be solved if the aircraft is a true FS2004 aircraft (freeware - most payware cannot be moved). Older aircraft - like the FS2002 C-182RG are simply incompatible.
Scenery - trickier because FSX completes the move to XML based coding which was only about 1/2 done in FS2004. Also FSX has a round world model where FS2004 has a flat world. Every one is different, even from the same creator there can be differences in whether or not the scenery is workable.
If you are asking about one specific plane, or one specific scenery - there might be someone here on the forum who has tried that item.
You also have to be careful about version of FSX. The differences between RTM, SP1 and SP2/Acceleration can result in an aircraft or scenery working for one and not another.
You might feel wronged by MS because FSX is supposed to be backwards compatible. It is, but unfortunately a great many of the gauges in addon on aircraft are FS98, not FS2004. A great many sceneries use FS98 and FS2000 coding which was never updated to FS2004 standards because it worked just fine in FS2004.
Last edited by ReggieF5421; 05-18-2009 at 02:09 PM.
@ PawPaw's house - near KADS, Addison, Texas, USA