Since, for some reason, my photos (which almost came out well, too) won't render when I preview this post I'll just list what I built for sake of clarity: five Skitarii Rangers with galvanic rifles, one with galvanic rifle and omnispex, one with plasma caliver, one with arc rifle, one with transuranic arquebus and an Alpha with arc pistol and taser goad.
Ease of Build
Okay, bit of an ancillary rant here but I need this for context: I bloody hate the bolters in the new Space Marine kits, they are a dryfitting nightmare to build. Who even though that making the three-part assemble less stable and harder to dryfit was a good idea?
Anyway, this kit does not do that with its guns, not on any of the builds: each gun assembly has two components: a large piece made of the gun and the whole right arm of the model and the second is the left arm that completes the assembly. Most of the guns are mono-pose but that's good as the reason for this is a groove under the armpit of the right arm that slots over one of the pipes on the side of the torso. It means you'll always get the right pose for the other arm to slot into without having to dry fit and practice. There were a couple of galvanic rifles without this feature and its absent on the plasma culiver as well but those guns are more forgivingly posed so that wasn't a problem.
The locating groove is part of the transuranic arquebus which I am immensely grateful for as I was worried the stand (which is a separate piece) would end up crooked if I didn't pose the arquebus perfectly straight.
Ease of Conversion
Since my first though was Renegades & Heretics conversions on these puppies I did look closely at how these could be converted. To be honest, if you want to swap the torsos out for something else, obvious option being the R&H conversion kits from Forge World, you're wasting some of the most characterful parts in the kit and I'm not sure the arms will fit them.
Head and weapon swaps shouldn't be too hard and I wonder if some sort of feral servitor or techno-barbarian close combat model could be made like that.
Take this as you will: I did some dryfitting and the spare head (the Vanguard variant) fit near perfectly on a Tempestus Scion and with a little cutting down of the ball join they don't look too out of scale with a Cadian Guardsman.
I have ten Vanguard heads and a few champion head variants plus various champion weapons options which will come in useful if there are HQ options in the Codex (or, touch wood, the full-size Adeptus Mechanicus Codex I anticipate will come after given the range branding). There are also the spare galvanic rifle and radium carbine assemblies though I'm not sure how useful they'll be unless they turn out to fit Guardsmen. There's also one radium carbine and one galvanic rifle just on their own, no hands attached, which might be useful for diorama or vehicle decoration.