As a newbie I have no history to know if this has been discussed, dissed, supported or otherwise beaten to a fine froth - I did do a search and found no thread titled with "windshield."
I ride an aluminum EZ Racer and love it. I am a solid 250 lbs and the EZ is a strong SOB. Climbs well enough and is very comfortable. Recently I put a windshield on it and have had some results which surprised me. It seems (and I say seems, because I am not sure) that it speeds me up. It has a significant effect in headwinds - I can crank one gear higher than I could without it.
Now this all could be on account of an increase in fitness; BUT . . . it could be a bit of assistance from the windshield.
Now chime in if you care to. Those that don't like windshields come ahead and express yourselves - I have one on my bike, I a not wedded to it.