Finally got a chance to put the new #8866 Train Motor and PF parts from the #10194 Emerald Night into one of my older locomotives, the #10020 Santa Fe Super Chief. There are some things you should probably know if you plan to do this.
The motor does not come with a PF wire, but you will need one. There's plenty of room for all the electronics inside the Chief, so #8886 is fine.
The motor comes with two black #3706 Technic Axle 6 as axles. As I understand it, if you are converting one of the newer #7897 Passenger Train or #7898 Cargo Train, this isn't an issue, because their Decorative Side pieces are drilled out to accommodate Technic axles. However, the old 9v #2871 Decorative Sides were only drilled out for the smaller metal pin axles. So, they can't accommodate the extra length of the axles, and can't be installed properly because of it. You have some choices to make it work. I went with number 3, until I can get the parts and try number 2. Use the decorative sides from #7897 or #7898, which should work but aren't available in Light Gray (old).
Use 5-stud Technic axles, which may not go all the way through the wheels on both sides.
Shave a half-stud length off of each 6-stud axle.
Drill out the metal pin holes in the old Decorative Sides to accommodate the larger diameter of the Technic axles.
[*]The signal strength between TX and RX is fairly weak, so much so that it has a hard time going through one layer of bricks. At first, for no particular reason, I yanked the brick-built engine out of the locomotive and put the battery pack/ESC toward the front of the interior and the RX toward the back of the receiver. Reception was erratic at best. I pulled the driver's seat and control panel out and put the RX there, behind the windshield, and the battery pack/ESC in the back. It worked fine from that point on.
[*]The motor has a bit of a hum to it. At higher speeds the train running drowns out the noise, but at very low speeds it's pretty audible.
[*]The battery pack/ESC and RX are both 4-studs wide, so they won't fit in narrower locomotives like the #10133 BNSF Locomotive. You could always put them in a follow-on car and run a wire to the locomotive, like the engine and tender in the #10194 Emerald Night.
I talked to a guy at LEGO Customer Service that's their train expert, and my call was the first he had heard about the problem with the axle length. There's not much chance of them making a 5.5-stud axle, but he said he will pass the issue along to the train team, so they'll know about it.
Overall, I still feel it's worth it, especially since it's only $11 (+ $3 for the PF wire). Just be aware there are some things you may need to work through.