My 2 cents.
My 550 DFC (3GX with red BL730MX) has been awesome. I'm certainly not a pro but it seems to be exceptional for the intermediate flyer with light 3D skills.
I bought it as my second heli as an upgrade and to cover up the fact that my first helicopter (TREX 450) was a little quick and got out of sight (loss of aspect) rather quickly.
The decision too go with the 550DFC PRO over the 600DFC PRO was mostly due to price point. The 550 is a 22V 6S 5000mah rig (I use a Haiyin 5000 40C pack), the 600 is a 44V 12S 2x6S 3300mah packs. The 550 runs on a 90A ESC with a built in BEC running at 6V. The 600 is a 120A ESC but requires a separate BEC and battery (you could go HV). The Talon 90 can be set to go HV, but I run the stock servos @6V with the 3GX and Futaba SBUS receiver (an extremely simple setup). IMHO, the 600 would out perform the 550 in a smack down. Of note, I have never seen a single brown out or problem with the Talon 90 ESC as has been noted around this forum in the past. turboomni was absolutely right about the head and the boom (they even say 600DFC on them, they are right out of the 600 kit). My flight timer is set for 8 mins. That gets me to about 40% TCS. I haven't done the final calculation to get me to the 20% mark but it should be just past that. I run an Idle 2 throttle curve that V's between 90 and 100% throttle. Stock 550 blades are used but there are some Youngblood 554s I would like to get.
The improvement with the 730MX is the KV rating that makes it more efficient with more torque at a lower temp than the 600MX. It is an advance (on paper), I can't say how much since I've never seen a side by side comparison.
I would say they're not being resold. My LHS has all of the crash kit parts stocked, but I too haven't seen any others at the field.
Haven't had the balls to auto yet, there is allot of high speed swinging mass in the head to give you a good shot at it.
I built mine with several HeliOption upgrades like the frame stiffeners, the main bearing block, and canopy attach hardware. There are plenty of build and setup vids out there as well. I've been flying my 550 since December and haven't had to do a single thing to it. I did however just upgrade the 3GX to V5.0 on the firmware tonight. We'll see how it goes (Align website said only 34 downloads for 5.0 as opposed to 44000 for the 4.0, I guess I stumbled on to that one).
Bottom line: It is a fine piece, The Honda Accord of Helicopters.