That's really a loaded question as there have been volumes written here in the forums on every possible variation. Here's my "down and dirty" take on it:
Most FS9 aircraft can be ported to FSX because all the word "porting" implies in it's purest form, is taking all the folders related to a particular aircraft as it relates to FS9 and dragging them over to the FSX equivalents. The one exception being that in FS9 you have a folder called "Aircraft" and in FSX it's called SimObjects with a sub folder called "Airplanes".
Now, once you've done that and loaded up your first ported aircraft, here are some things you may discover:
ver.'98 gauges do not work in FSX
Prop discs sometimes appear as solid discs rather than the nice prop blur you expected.
Some props, even if they appear to be transparent may knock out all scenery objects and clouds when you look through the prop blur
If you attempt to use the DX10 Preview option in FSX, you'll find that many FS9 portovers appear white (no textures).
Depending on how the author coded the materials used in the model, you may have panels that don't show in the VC or the VC may be transparent except for the gauges.
VC's that were clickable in FS9 may not be fully clickable in FSX
That's just a few, I'm sure as folks wake up and get back into the forum, you'll have more than you ever expected. Some of those I've listed are fixable, some are not and depending on how many of those happen to show up in a particular aircraft will determine whether or not you keep it or dump it. Personally, I have more portovers than I do native FSX aircraft, so the problems I've listed aren't as widespread as it may sound, but they are out there.
Ok, that's my 2cents. . . .let the flood gates open and read what everyone else has to say.:salute:
