Heh, mids are actually probably the best wonder in terms of time-frame and pay off.
They are accessible at Masonry, a tech some civs can tech on T0
They grant access to Representation as early as the T60s or 70s, depending on how hard you push. Normally requiring a mid-game tech, you get +3 happy in 5 cities and it basically doubles the value of Libraries and scientist specialists. Extremely powerful bonus that early.
Can be used to farm Engineer points. Said GE can be used for bulb (such as Machinery), used on another good wonder (Taj would be first choice) or held onto for anything else like Mining Inc.
Lets you actually use Police State in an acceptable time frame instead of waiting for fricken' Fascism. While not that useful IMO for later war like whipping Cuirs where you will already have/be quickly gaining cities, I've done some sick Elepult attacks with it before, especially if combined with a good HE city. It's like a catapult per turn or elephant per chop then.
And lets you swap to US without ever needing to bother with Democracy. Pretty nice if you went to Communism fast and want to swap into SP + Caste.
All that said, it's not a wonder I'd consider a must (try to) build. If it's feasible, Mids are fantastic, sure. But they are expensive enough to really cause one to weigh the merits of trying for them or just focusing on more standard play. I've chopped them without IND or stone. Wouldn't really recommend it though.
Only wonder I will try for (or at least evaluate if it's possible to get) in every game is Taj...and maybe the NE, but that doesn't really count as you can't be beaten to that. The free GA is so handy, especially properly timed to coincide with a civic swap over or to extend a previous GA. (plus 1 free turn of GA if chaining them with Taj). I love to swap into GPP mode with a 1GP golden age, (Music GA, GW spy, Mids GE, or just random GS/other GP lying around) start up Taj, finish it for a second round of GPP farming and then end it by swapping into war mode civics.
Other wonders that are situationally very powerful are GLH and Great Wall. Island heavy maps with GLH is like printing free commerce with every city you settle. Huge maps with large landmasses, or if isolated with a lot of land, GW can change the entire dynamic of your game.
And then the "it's a nice thought" wonders. All great but not really all that necessary whatsoever:
MoM
GLib
Notre Dame
Statue of Liberty
Colossus
Kremlin
MoM is a really great wonder, too bad it's so highly sought by the AI and there's often little reason to rush Calendar (often a trade back tech).
Great Library is really cool to have, especially as a PHI leader, but it doesn't really achieve anything you can't with good use of starving GP in a golden age. Also really like to have Marble for it, especially because pairing it with NE in a high food spot really compounds the boon it grants.
Unless you Engineering rush good luck beating the AI to Notre Dame in the first place. Global +2 happy is a tasty boon though
SoL is in the same vein. Nice wonder, bad placement in the tech tree and AI really likes going for it. It's also like, crazy expensive, and without a dedicated build effort the AI can catch up and overtake in the race for it.
Colossus is a nice little wonder for FIN or IND civs if you have a lot of water around. Metal Casting is a tech needed on the way if you are planning to put a bulb in Lib and forges are nice buildings anyway, so it's not such a detour. It rarely seems to work out that way for me though as at least one AI in the field tends to go MC early and get the jump on it.
Kremlin has a really nice effect for sustained war, especially so if in US and rush buying (like with Tanks in late game...awesome) It's my 2nd favorite wonder in the game, and tied to an excellent tech. Highly dependent on what the AI is doing though, Communism gets teched with a lot of preference because of the GS bonus for 1st to research.