It's very, very stupid. That's not a criticism. That's the basis of its humour. I wanted to like the show and not infrequently I did, but good grief.
The premise is that everyone has a secret. Asahi Kuromine's is that he's in love with his classmate, Yoko Shiragami. Other secrets are a bit more exotic, since this show includes a vampire, a devil, a werewolf, a pair of talking glasses and a tiny alien piloting a human-sized copy of herself. Asahi's ordinary human friends have no idea that there's anything unusual going on. Apparently the manga has still more wackiness, e.g. a time-traveller trying to avert a sex-mad dystopian future.
The genre is romantic harem comedy. Asahi's only got eyes for Yoko, but unfortunately she's dense (albeit also sweet) and thinks he likes Aizawa instead. Meanwhile much of the cast is either secretly in love with Asahi or more simply want to have sex with him. Oh dear. One last twist is that everyone seems to be stupid and/or embarrassingly bad at telling lies. In Asahi's case, the latter is his defining characteristic. Everyone can read him like a book. The result: lots of comedy based on people behaving like morons and overreacting in attempts to hide something.
I'll admit it. This can be funny. This is an endearingly good-natured series with lots of energy, so it does sometimes achieve inspired things with idiocy. However more often it's just silly, with the harem angle not helping either. Silliness is the point. This show exists to be silly. If the anime might ever seem to be getting slightly more serious (e.g. ep.12), you can probably assume it's diverging from the manga in order to do so. The results aren't really to my taste, alas. Certain elements of this show I like a lot, but unfortunately the overall daftness pulls it down into an amusing sludge of "uh-huh". Would I watch a second season? Probably, but I'd be happier if the production team went off and made something good instead.
Expect confusion about the show's title, by the way. "Jitsu wa Watashi wa" literally translates as "Actually, I..." Crunchyroll used "Actually, I am" for the streamed anime, while the manga has been licensed for English-language release by Seven Seas under the more marketable title "My Monster Secret". "Jitsu wa watashi wa" is actually a key phrase in the show, getting said a lot by characters at important times, but not all translation points are worth killing yourself over.
STUFF I LIKED
(a) Asahi and Yoko. I genuinely love ep.1, before the rest of the gang's shown up. Yoko's sweet and there's some emotional underpinning for her need to preserve her secret. Her Kansai accent is also lovely to listen to. Asahi's inability to confess properly to her does fall under the category of "plot convenience", though.
(b) The emotional depth, when we get it. It's not the show's focus, to put it mildly, but it's possible to care about Aizawa and Mikan once we've seen what makes them tick.
(c) Akane the devil, especially in her antagonistic relationship with her great-great-granddaughter. What's refreshing about Akane is that she has no romantic interest in anyone and genuinely lives to be evil. In a series where vampires and werewolves are completely harmless, Akane's devilry is surprisingly full-blooded. In the very, very funny ep.8, she decides to destroy the Earth in a fit of pique at not being able to eat some chocolate. (Her great-great-granddaughter then trumps this. That was another good episode.) She's also childish.
(d) The running gag about alien technology and satanic magic actually being a bit rubbish. "This is not a hammer! It's a memory erasure device!"
STUFF I LIKED LESS
(e) Mikan the sadistic journalist feels too much like a plot device for my tastes. "It kills all the fun when you enjoy it!"
(f) The werewolf. In her female form, she's a one-joke character that isn't very funny to begin with. (She's a pervert! She doesn't always wear panties! Um.) Her male form is never seen again, not even in the finale when they're sneaking around in the moonlight. He's male, you see. Male characters are superfluous in harem franchises. For example, there's another male character from the manga (Nagisa's brother) that the anime simply omits entirely.
(g) The fact that it's a harem comedy in the first place. Yeah, I know. I might as well complain that the sky is blue. But still.
Would I recommend this show? No. It's empty calories. Its daftness can be funny, in fact at times very funny. I'd probably watch a second season. However it's the kind of show where silly plot contrivances are in fact the plot and the characters will never stop acting like idiots. I can see why it's popular, though.