This is three seasons across two years. I'd have normally broken that up into two or three reviews, but that would have been pointless here. There's not enough content enough here to sustain three reviews. Well, there's not actually enough for one. It's a bunch of throwaway semi-comedy shorts starring the idol characters from an idol anime based on an idol-training computer game. Personally I loved the parent show (THE iDOLM@STER: Cinderella Girls), but even I couldn't recommend this.
It's never bad, though. Occasionally it's funny. It's just that it's a bunch of random throwaway scenes that don't go anywhere or achieve anything beyond, at best, mild amusement. An extra problem is that Cinderella Girls has a huge cast, even more so than usual with iDOLM@STER, and I'm pretty sure that these Theater mini-episodes will happily use any and all of them. If you're currently watching Cinderella Girls, you'll know who everyone is. If you only watched it even as recently as a year ago, though, forget it. Character-based comedy is less effective if you can't remember the characters. Personally, I'd recognise Kirara (the tall, lovable one), Anastasia (the Russian one), Ranko (the mad chuunibyou) and Anzu (the lazy one), but after that I'd be a bit lost.
If you've never seen the parent show, get out of here. You'd be hopelessly bemused if you tried to watch this show. Barely distinguishable girls do cute, silly and sometimes absurd things. Most scenes only last a minute. It's light and friendly, but there's not much point in it.
It's not even as if this is the first short-form iDOLM@STER comedy spin-off. Puchimas! Petit Idolmaster was far better than this, although way, way more absurdist. That show gave every girl had a semi-sentient Puchimas clone of herself, the size of a guinea pig. They spoke in an incomprehensible babble, they behaved like toddlers and they created a clear, funny situation on which you could build distinctive mini-episodes. That show also gave the Producer a letter "P" instead of a head.
That said, though, I watched all of this show and quite enjoyed it. It's amiable. It doesn't often hit the heights of "Actually Funny", but it's comfortable at the level of "Good-Natured And Painless." It's easy to watch. You put on a little episode and then before you know it, it's over. You might continue with another one. Besides, the show would occasionally make me laugh, e.g. Anzu's approach to commercials in web-ep.9 or the idol who goes on a roller-coaster in TV-ep.29 because she thinks it'll be an opportunity to look scared and be girly. Other funny episodes (for me, at the time) included web-ep.3, TV-ep.4 or TV-ep.7.
The show's threatening to head into harem territory. Everyone's in love with the producer and we never, ever see his face. (This is a shame partly because the producer in the parent show was a pretty good character, but of course he's an anime avatar for the original games' protagonist.) That said, though, the girls' thinly-veiled romantic intentions can sometimes be funny.
There are also lots and lots of end credits sequences. They seem to do a new one every few episodes and you'll have your favourites. (For a while I thought nothing would match the first one for me, but "Akimeite Ding Dong Dang!" came close.)
It's probably the ultimate in low-effort anime (from a viewer's point of view). It's got no story. You don't need to keep track of the characters, because you probably won't be able to. It's never heavy or hard work to watch. It's just light, fairly empty fun. Lots of cute girls messing around and being silly. Don't expect to laugh out loud, but it'll probably raise a smile from time to time. That's about its limit, though.