I doubt I'll ever play it because it's so overhyped, but it seems fine.
It
is overhyped. You quite literally have to play the game twice if you want the "pacifist" or good ending. It's not a bad game by any means but a lot of the characters are very one-note and the game-play is usually repetitive and boring. DeltaRune is arguably better but it's a part of an incomplete series of games, apparently.
Undyne is alright. She suffers from the same issue of seeming one-dimensional, imo. The most interesting thing about her character is one that we didn't even get to see in anything but text form (i.e. that one neutral ending where she overthrew the queen). The worst thing about her is her taste in women.
Alphys (Undyne's girlfriend) is essentially a less likeable and more unhinged version of Otacon from Metal Gear Solid. Instead of her crimes against the living beings around her being a consequence of an invention (in comparison to Otacon), she directly experimented on others and permanently mutilated and even
fused individuals together. Instead of owning up to her crimes immediately, she instead locked her victims up in her lab and hid them from fear of having to, y'know... face the repercussions. When she did let them roam free (which she had to be convinced to do by a small child), she faced zero punishment for it. None. Nada. Apparently, just letting them out (something she should've done in the first place) was seen as being a good enough redemption. And she got her dream girl to boot. Awful.