"Darth Vader went together fairly easily. The Technic torso seemed a bit over-complicated for what it does, and had a few pieces that seemed unnecessary, but otherwise looks great for the most part.
The back is way too unfinished in appearance and only the cape makes it forgivable.
The cape is tacked on as an afterthought. In the movies, it is attached with a chain around the neck, however here it is just held on with a Technical piece that looks bad enough sitting out there by itself, and that's not considering that the top edge of the cape is just floating at the back of the neck. It also has a hole as if at some point in the design process it was fitted around the neck and attached at the front beneath the upper chest piece, but now looks like a mistake.
It would work much better if the top of the cape were extended upwards and the two small attachment holes placed there, with two larger holes vertically below those. The cape would be fitted upside down and locked in place as it is now, then folded back over so the neck passes through both larger holes, and only then flowing down the back. If the big holes were sufficiently large or even a single ovoid cutout, then an actual chain like the one in the photo attached (from one of my daughter's Friends sets) could be passed through and around the front of the neck and make it look vastly better - especially with a much better proportioned head.
The head, while very well sculpted and having a silver nose and chin points as a nice touch, is disproportionately small. Did the designers forget that there is supposed to be a human head inside this helmet? It needs to be 40-50% larger to match the rest of the figure.
Other minor quibbles include the grey ankles and thigh bones - these ought to be black, and on the feet, a Technic blue pin with a half top and a 1x1 slope piece or similar would make the foot look much better. Such a pity there weren't enough spares to do both feet.
The articulation of the figure is a bit stiff and restricted, but that isn't necessarily a disadvantage - in the movies, Darth Vader was crippled by the time he started wearing the suit, and most good rigid body armor restricts mobility to some degree anyway.
The use of a red + cross-sectioned peg to provide a left-handed grip is a big No from me. It restricts the grip too much. Far better to use a narrow (1.5mm?) round dowel so that the lower hand can "grip" the bottom of the lightsaber at any angle without having to bend the wrist. Again, a pity I don't have one.
As for posability and play, the figure is very heavy in the torso and restricted in its range of movement, but as I said earlier, none of this is necessarily a disadvantage - Darth Vader was supposed to be crippled and these limitations only serve to emphasise this, enforcing the more balanced and less energetic poses and good posture that would be expected from an old martial artist with multiple disabilities."