I was lucky enough to be purchased this model, and was really looking forward to building and painting it. The Onager is an awesome kit; a really nice model which forms part of the (pretty small) Mechanicus range...
I found this model to be a lot of work, but in a nice way. If you're a "buy a box, build the model the same day" kind of guy, it'll be even more work. I went with careful sub-assembly and painting by sub-parts. Because the two main colours - the deep red of the hull and the bleached bone of the legs - are so different, painting the piece in a single go would be prohibitive. So I left the legs off, as well as the armour plating, and concentrated on building up the bone with multiple layers. I then used the Khorne Red spray paint on the main hull, which saved a lot of time.
My favourite set-up is probably the neutron laser - that gun is so very 1950s sci-fi - but in-game, the Icarus array is too tempting. I always struggle with anti-air (my regular opponent always brings two Stormravens, and is talking about a third...).
The set comes with a small decal sheet, which is actually packed with iconography. It was nice to see a sheet which doesn't feature the same old Space Marines transfers! The white icons really off-set nicely against the red here, and add some extra character to the model.
I had previously decided to use Martian Ironearth to base all of my Mechanicus models... Now I'm not so sure about that, but I'm probably committed! The Ironearth is very flaky, and I'm not sure about durability. Maybe a couple coats of varnish will improve things. If you do use Ironearth, note that a pot doesn't seem to go very far (I've based this model - admittedly, a larger example - and a couple of Kastelans with the data smith so far, and the pot is virtually gone), and it is quite translucent out of the pot. I painted the base Khorne Red first, to lay down a foundation for the Ironearth.