I want to say a few good things about the Kremlin. On paper, half-off rushing cost might not sound like much, but it is actually quite powerful in practice.
It should also work for slavery civic, theoretically, something that requires 4 pops now requires 2 pops, although I have never tried it.
The effective period is not the longest, but it pretty much lasts until the end of the game since fiber optics (i think that's the tech that obsoletes it) is near the end of the tree.
One of the best uses of the Kremlin is to rush wonders. Some of the wonders available in the effective duration are: Pentagon, Three Gorges Dam, Eiffel Tower, Hollywood, Broadway, Rock'n'Roll, United Nations, Space Elevator... A lot of them are popular on this thread. With the help of a healthy economy, universal suffrage, and the Kremlin (and being industrious) most of these wonders can be bought by zero out the tech slider for 1-2 turns (no more than 2000 gold needed for each, and setting the slider to zero gives me at least 1500 a turn. except for the Three Gorges Dam, that one is a bit more expensive, i think i spent 3000 gold to rush one that is 1/3 completed.)
Another use is to quickly build up a modern army. If you have a lot of unpromoted units it would be cheaper to rush a brand new unit than to upgrade the old ones.
And of course it makes rushing general buildings easier. Rushing buildings to culturally protect my oversea expansions became a lot easier with the Kremlin.
Overall I think the Kremlin won't affect the game as much as some other wonders, but if you are a wondermonger, or maybe you find yourself with a lot of cash and behind on production, then the Kremlin can really make a difference.