I had very low expectations and ample warning for this show, expecting only said breasts and panty shots with a modicum of light storytelling to carry them. Unfortunately I was to be disappointed even in these very qualified expectations.
What Popotan is about doesn't matter - the show makes this very clear early on. Yes, there's a larger story about connecting with others which is given undue seriousness for such a light piece of entertainment, but what underpins it is a plot so half-baked you're risking salmonella poisoning by watching it. It's not even clear that the characters are time travelers until about halfway in, but by that point you've likely learned to take the show's storytelling in stride (or better yet dropped it). However, things just get more confusing from then on, with dramatic scenes falling laughably alongside gratuitous nudity for some extra tonal dissonance. Where did these characters come from? Who is the person they're looking for? Why do only two of them appear to be able to use magic? Why does Mai try so hard to befriend an anti-social girl when she knows that she's going to disappear and leave this girl alone, repeating the exact thing she now deeply regrets from a previous point in history? None of these and more will be answered.
Sadly, for those who can suffer a terrible story for some decent fanservice, the art of Popotan does its best to disappoint here as well. While the character designs themselves are distinct and cute, in motion here they're mediocre at best and sometimes reduced to blocky geometric shapes. Certainly some of the best animation is reserved for the frequent bath scenes, but even here the show fails to deliver consistently. On a side note - it's somewhat remarkable that for such a brazen work that features all of the characters with detailed upper-body nudity in every episode (often more than once) it's surprisingly skimpy when it comes to Mii.
As for positives? First and foremost is likely the capable voicework of Momoi Haruko as Mii, who is just as nauseatingly (enchantingly?) cute as you'd expect. You also get some nice early 00's atmosphere and a decent soundtrack. The fanservice is also decent when it wants to be, but is probably not worth the price of admission.
