There is a phenomenon known as "recognition humor", wherein the humor of a joke (gag, skit etc.) lies not so much in the wittiness or inventiveness of the joke as in the fact that the reader (viewer, listener etc.) recognizes the situation, mood or characters of the joke from his or her own experience. Several comic strips have been based on this; for example, many of the early American newspaper strips went for the family setting for that very reason, and the successful Scott - Kirkman collaboration, "Baby Blues" started out very much in the "something for all new parents to recognize themselves in"vein even if the characters would later develop individual characters that would elevate the strip beyond that (much like Scott & Borgman's "Zits" or any other really good strip IMO).
Anyway, "Medelålders Plus" is about a man who suddenly realizes that he's retired, not just middle-aged, and his crosswords-solving and flowers-growing wife. They're a happy couple, not making any big waves and rather happy about that, vaguely worried about encroaching old age but not particularly anxiety-ridden. In short, ordinary people.
And ordinary people seem to like the strip. A lot. Me, I don't know how ordinary I am (but not being particularly extraordinary, I suppose I am, at that), and while neither "middle-aged-plus" nor in a happy, stable relationship with a crosswords-solving and flowers-growing wife, I do know and have known a few people who remind me quite a bit about the couple in the strip – relatives, and friends of my parents. So there is still some element of recognition there for me. It's not quite enough to make me a fan of the strip, but it is enough for me to understand why many people are.
For me, the strips can be sorted into three categories: laugh-out funny, amusing, and those that just sort of pass by. The laugh-out funny ones aren't all that frequent, but after a lifetime of reading comics, not many are. Still, there's something very funny about a recent retiree standing by his window annoyedly looking out at some workers digging a hole in the street obsessing about their low productivity, yelling advice and exhortations that they can't possibly hear, and in the final panel having to go lie down a while because he's become all stressed out. Similarly, it is amusing to read about the social interplay and relations that develop around the recycling center, where people regularly meet who otherwise wouldn't even know the others existed. Etc.
I'm not all that big on recognition humor myself, so "Medelålders Plus" isn't quite my cup of tea – those laugh-out loud strips don't come often enough for me – but it's a likable, low-key strip that gives voice to a demographic that certainly isn't particularly well represented in either comics, movies or the news, so more power to it for that.
Article in Swedish about the strip here; samples (also in Swedish) here.