Update, for the curious. It's a Pilot Metropolitan Pop and a Midori MD notebook.
FWIW, of all the entry-level fountain pens I've tried so far, this Pilot MP has my favorite weight and balance.
I'm going to live with it a little longer first, but if it still feels this comfortable after a week or two, I may go ahead and get a second one. Maybe the red.
Or the green.
Or the black.
I don't know.
So far, my big complaint re: the Pilot Metropolitan Retro Pop is that the squeeze convertor doesn't seem to hold ink well. I don't know if it's losing ink or drying up or what, but if the pen sits capped for a few days, I have to refill it to get it to write again. I'll look around for solutions (user error is a definite possibility) and maybe try a different convertor. Hopefully, that'll resolve the problem, because I really like the form factor otherwise.
If this works, then my biggest complaint is that it only works with proprietary cartridges and converters. Not a huge deal, but compliance with standard fits would be a whole lot cooler.
The bulb convertor on that first Pilot pen just wasn't cutting it, so I ordered a piston convertor. While I was at it, I went ahead and threw in another Pilot and conveyor. The notebook was just an easy way to bring the total up to free shipping level, but I actually like it. A lot. The Apicas are turning into my favorite line of notebooks.
Okay, so user error may have been the major cause here. Turns out I wasn't pumping the squeeze convertor enough. You're meant to pump it until bubbles no longer form in the ink, then let it slowly draw until full. Previously, I wasn't seeing bubbles at all because I was compressing the bulb BEFORE I submerged the nib, and only letting it draw once. It's possible that what little writing I got out of it each time was just the ink in the nib. I've filled it properly now, so we'll see how it goes.