I can't compare the Loom to the Safari or Metropolitan, but I like the Loom enough to own three of them, with another on the way. All the nibs are great, and interchangeable among pens in the Faber-Castell Design range. I have the EF, M and B, too – felt like I wanted them all, as they're all so good – but I particularly like my F nib, which I have in an Ondoro rather than a Loom. It may also have something to do with the higher-priced pen, which feels better in my hand than the Loom, but right now I reckon my handwriting looks better using that than any other pen.
Ink ... I've used Noodler's Blue-Black, Visconti Blue, Graf von Faber-Castell Cobalt Blue. Very different inks, but all have played nicely with the Looms. They're quite wet nibs, though, and from an M nib the Noodler's Blue-Black put down an intense line that took impractically long to dry. Maybe it does that with other pens, too, but I couldn't say. It's irritating because I like the colour.
Downsides ... the section is a bit weird and my fingers sometimes aren't sure where to grip it. The section on the Piano version has a better gripping surface than the metallic version. The cap of the metallic Loom (I only have one, so it may just be mine, but I've heard similar reports on here) is really tough to pull off. Annoyingly so. And it hasn't improved after a couple of months. So I'd recommend getting the Piano. It looks better too.
You can post them but I tend not to, as the bottom of the cap ends up in a position where it irritates the skin between my thumb and index finger. No such problem with the Ondoro.
I haven't see any plastic sections. My most recent Loom, ordered a couple of weeks ago (in Germany) had the usual metal section.
In general, good pens and great nibs for the price.