You know, that I play with Settings far from normal, but I've found the Mids actually always been worth the hammers but never as op as many people make one want to believe.
I. e.: Extra Happiness: Doesn't matter, just build a few more Warriors, and HR is even better because it's cheaper and one can have as much happiness as one wants in a large city, and as few as one wants in a small city.
Extra Beakers from Specialists: Those make up for the lost hammers. Lately, I find myself only running Specialists during GAs and then in 4-5 cities like 10 in each of them. Then, the extra output really matters, but again, the hammers invested could have been something else aswell.
PS: Is not important in my games, as I'll have 20+ cities of which ony 2-3 build troops, it wouldn't be worth the extra upkeep.
But: Early US without having to research it can be extremely useful in my games, as I reach the part there where I've reached Sushi and don't want to research anything more than necessary. Then the mids are 16k Beakers and that's why I try for them in most games. Also, the GE-Points, together with the ones from HG in one City + an Engineer Specialist help to put out a GE to rushbuild the extremely expensive Wallstreet / the Kremlin.
Those are very specific tactics, so I come back to the beginning, Mids are good, but they're not as good as most people say imho, at least in my games with many cities. I can see their use for smaller Empires, and for Tactics like cseanny described. Working REP-Specialists though? Have tried that, wasn't better than building Cottages or even Mines. Don't know how it is on normal speed though or on smaller maps. Might also be better with not being FIN, it is probably. I imagine if one plays Rome or some other ORG Civ, all those Spy-Specialists might come in real handy with REP, also supporting a playstile with huge military conquest where one is usually backwards and has little to no tech-trade friends.
GLH is 10 times better though. I quit the game if I miss that
Btw.: On Marathon, The Pyramids go "between 2000 BC and 1000 BC (T 150-250) . Sometimes even later, sometimes nobody builds them, sometimes they go so early that one would have had to invest everything just to get them. Very unrealiable Wonder to fit one's playstyle to it imho.
And to the thing with stacking multiple different GP points into one city: I find it very frustrating when the wrong kind of GP is born, also, 4 cities produce more GPs than 1, so I tend to split the pools by their type. This gives me a 2nd GA from the MoM, which I also find way better than the Mids, to start a 2nd GA, gives me a GM from the GLH / Colossus (normally Trademission) , a GS from the GL and a great Prophet to build a Shrine from the Oracle, and I know that those persons are safe and that I can count on them. Mixing up the pools has always ended in getting a useless GP that had like 1% chance, or in getting a GE, that is worth too much to be used for what I had planned at first so I have to wait for the next.
Btw.: Actual Record of GP Creation in one of my games during the 1st GA: 6 GPs running 5 GP Farms! In the next GA it were again 5 GPs and after that 4, so those were almost 20 GPs with the free ones from Music, Communism and Physics. 3 of those were GEs btw

But, again, not really on topic as most of it is too specific and only for Sushi-Approaches on Huge maps.
Cya