I think Scubapro's premature discontinuation of the D400 and replacement with the not yet ready X650 in hurt the reputation of both SP and the X650, particularly given that it came on the heels of the freeze problem of the Mk 25/S600 combination.
However, I bought an X650 after the last round of upgrades and it has been totally reliable so I would not shy away from buying one on that count. While it is not in the same class as the D400 in terms of super smooth and easy breathing performance, it is equal to the S600 and even shares one of the same limitations.
Both the S600 and X650 in my experience display a very slight freeflow between breaths in very cold water (under 40 degrees). In my opinion this makes both the S600 and X650 a poor match for the TIS/AF equipped Mk 25 in very cold water (again under 40 degrees) as that slight freeflow over the course of a dive will increase the cooling of the first stage and create the potential for a first stage freeze up. In cold water with the X650 I found I had to increase the cracking effort with the adjustment knob to 2.0" of water to prevent the slight freeflow. (And in that case, you are going to get better inhalation efforts from an R190 that costs 1/3 as much.) Oddly however, the X650 does not display the problem in warm water.
The X650 also does not share the same coaxial exhaust valve/diaphragm design as the D400 so it does not have the same case design fault advantage as the D400. Where the D400 will offer the same very low breathing effort and freeflow free behavior in any attitude without any adjustment, the X650 performs more like a conventional second stage design like the S600 or G250 in that some adjustment is required to get peak performance in different positions and to avoid a freeflow out the exhaust valve in unusual attitudes.
I think the in water performance of the purge cover on the X650 is comparable to the S600 as a squeeze anywhere across the front of the cover will purge the reg.
But overall, I don't see the advantage in having an X650 as the angled diaphragm offers no real advantage when it is divorced from the coaxial exhaust valve. My guess is that the X650 is going to be relatively short lived design as it lacks both customer and dealer support after the two redesigns and due to it's lack of any substantial advantage over more established second stages. Scubapro could do a fairly significant redesign of the diaphragm, lever, exhaust valve and purge cover to incorporate a coaxial exhaust valve diaphragm design that would turn it in to a superb regulator, but I doubt they will. If they did, it would probably warrant a new name and would in fact benefit from one to distance itself from the X650 reputation. (X750 maybe?) The saving grace for X650 owners would be if the upgrade were retrofittable into the X650.
At present, my suggestion is to get the same or better performance by going with a G250 HP rather than an S600 or X650 and save a fair amount of money in the process.
With regard to first stages and very cold water, if you are diving in water colder than 40-45 degrees, a Mk 16 is a safer bet than a Mk 25, as the Mk 16 is very relaible in cold water. In warmer water, the Mk 25 is great.
Personally, I still prefer a Mk 20 with the older filled ambient chamber and SPEC boot for environmental protection. In my rather conservative opinion, Scubapro would benefit from introducing an optional SPEC cap and boot with an O2 compatible filler for the Mk 25 to meet the needs of divers who do dive in what SP considers to be "extreme conditions" rather than just ignoring problems divers are having in very cold water with the Mk 25. The sad part is that many of the strongest and most loyal customers Scubapro had were divers who dove in extreme conditions and consequently used SP equipment as it was utterly reliable.