I got mine from a friend who manages a sunglass hut type store.  He swore by it and I was like .... yeah yeah .... until he treated my glasses with this.  This stuff is like Rain X for glasses.  Apply this over both sides of each lens with the pen thoroughly and let dry.  Wipe until clean and clear with the cloth that is included in the base of the pen.  The bottom cap holds the cleaning cloth.

This keeps your glasses waterproof and dustproof for about a week, maybe longer and make the lenses look new again.  This stuff is great.  If you watch at sunglass stores this thing goes half price like 10 bucks every now and then.  Either way, at whatever price, this stuff is worth it if your glasses get dirty and/or wet a lot.  Besides making the glasses look brand new it makes cleaning easier and faster too in between applications.  I have gone on to buy 2 more but only when they were on sale for ten bucks.  I had a total of 3 because my pal gave me the first bottle pen.  I'll get more (on sale) when I run out.

Disclaimer:  Just because I compared the action of this hydrophobic pen to that of a car product does not mean I suggest or endorse that you even consider trying to use that.  If you damage your expensive glasses messing with that stuff then it's your problem and fault.