The Plexus does a nice job of making the headlight look clean and sparkly, but it is only a polish and doesn't remove swirls, scratches, haze, etc. I have two cans of it sitting on my shelf so this is personal experience talking. What does work well is the Novus 3 step polishing system. (also on my shelf). You can use it by hand or with a buffer, but the comments about the plastic returning to its previous condition are completely correct.
For a longer lasting solution, speak to this guy:
Headlight restoration service
The owner's name is Richard Cheatwood. Apart from being a satisfied customer, I have no other affiliation with this company.