Application is very simple, but I found the brushes supplied with the product to be rough, unwieldy, and wasteful. They remind me of the paintbrushes that come with kid's crayola watercolors sets. My esthetician instructed me one drop per eyelid each night. After the first couple of times, I trashed the brushes that came with the product and have been using a narrow MAC eyeliner brush. I rinse it off after each application and am not worried about infection as I am the only one who uses the brush. My brush allows a much more precise application along the lash line where the product is intended to be used.
I have had some mild hyperpigmentation along the lash line along with some irritation and redness of my eyes in the morning. Sometimes if it is bad, I will skip a night or two. Mosly, I just use some Rohto eye drops in the morning and that clears up the discomfort and redness. I have green eyes and have had no color change in the iris as is often mentioned as a side effect. When I researched this, I found that this side effect has only been documented in people using this product for glaucoma which involves putting the drops directly into your eye and not just along the lash line. This warning is required by the FDA though.
Now for the good stuff: results! My eyelashes are definitely thicker and longer than they have been in years. This product has absolutely worked for me. I do still use mascara but now it really seems to have that "wow" factor that I've not had for awhile. I also use an eyelash curler and it seems that the lashes are stronger and less likely to come out when I use it.
Only cons for me are that this product may cause some discomfort and it is expensive. I read a tip to put one drop into the cap of the bottle and dip your brush into that to use less. I've found that by doing that with my good brush, I can do both lashlines with one drop vs two. Overall, I would definitely recommend this product to anyone who is concerned about short or sparse lashes.