New here...and new to CIV, so what I say will probably be 'disputed' by some of the vets...
I used them my first time on Regent ... I was far enough ahead in techs that I felt I could afford 4 turns to research them. (Me Byzantines, democracy, Huge map, 15 Civs, 70% Continents, 3 bill earth.) Allowed me to put off going for a modern navy via steel/refining/combustion branch a few more turns and go instead for TOE and Hoover.
Good thing...I have used them to keep my lux flowing without paying a single gpt. With market's in most big cities I have not yet raised my lux slider above 0. I have traded them to 4 other civs so far, getting 4 luxs for an additional 20 turns cheap. (I have not allowed any of those Civ's iron and/or coal - I am the only civ with extra of those to trade so they cannot build them against me.)
I also immedialtly built 20 ic's, using them as escorts as the Spanish were decimating my frigates and galleon convoys to the other continent with numerous privateers and the English had many Men 'o War. Worked great. I was able to keep my galleon chain to the other continent safe to support me against the Romans and Sumerians. I was also able to research for 2 or 3 turns with the sci slider at 0 to build up a substantial treasury...no pressure to get a modern navy any quicker.
When I was ready to build a modern navy I put my ic's in port the turn before getting Combustion and viola!... in ONE turn I had 20 sleek new destroyers pulling at their leashes. Lincoln who had built ic's declared against me a few turns later ... to his surprise and regret when my new destroyer's accompanied transports which debarked an Army on his shores, and whupped 5 of his 10 ic's in the process.
I keep reading here that this tech or that tech is useless... but in my limited experience with some creativity and imagination each tech can be useful ...and each tech could also be worthless ... depending on the game environment. IMHO it depends on the situation and how one decides to play the game. Don't write off a horse until you check out the track...