Appearance & Design:
I really like the way this pen looks. It is some kind of crazy design from outer space, but it does draw the eye. When I use it people notice, and I'm into that sort of thing. If you want to be subtle, then this isn't your pen. The over-sized cap and pocket clip are almost certainly the thing you notice first, and they come in lots of different color combinations ranging from my old white and black model to a retina-burning red and orange combo.
Construction & Quality:
The barrel and grip section of this pen are made from a lightweight plastic that just barely avoids feeling cheap. There is no flex in the plastic, but it's so light that you might feel that you'll break it. I can say that I've carried these Cores for well over a decade and they haven't suffered so much as a nick or scratch on the barrel. They're tougher than they appear.
The cap is a much heavier plastic that is mostly covered in a soft and smooth rubber (the grey areas you see in the pictures). The rubber is perhaps a bit too soft, if anything. The pocket clip is a big and fairly stiff, and the combination of a stiff spring and rubber coating means that just a little bit of the rubber under the spring will be chaffed when you clip it to, say, jeans.
Nib & Performance:
Rotring marked this nib "XS", and I'd say that it puts down a fine line. It's not as fine as the line you get from a fine-nibbed Hero, so it's fine enough to see some nice effects from your ink while not worrying about it bleeding through the page or blending your letters together if your writing is small like mine.
This is actually one of my most consistant writers. I've used it with many, many different inks over the years, and it has handled everything from thick Private Reserves to fairly thin Noodler's Habanero with aplomb. It doesn't clog or get out of control regardless of what I've put it through. It has never skipped as long as I've had it.
Filling System & Maintenance:
The Core takes a cartridge (tall or short) or a converter. Mine has only used the Rotring piston converter, and it has never leaked or come loose.
I've read that you can remove the nib and feed from the pen if you need to, but I have never tried. Now that the pen is out of production I don't think I'll attempt anything drastic unless I absolutely must.
The pen rinses fairly well, but there is some extra work involved in cleaning out a pen when you can't remove the feed, and so I don't change inks as often as I do with an Ahab, but it's no more annoying than the Lamys that I have.
Cost & Value:
I really like this pen even though the design is weird and some will dislike the saddle-grip design of the section. I think these used to be around the $20 mark, but the prices are all over the place now that they're not being made. I've seen them for up to $300, and that's preposterous. It's a good pen, but that's beyond the pale. I'd say that if you can find one for $30 or less you should snap one up. Darn Rotring for discontinuing this line (and almost all of their FP lines). I would really like to get my hands on a couple more of these.