I would make love to this if it were a person.
I love it so much because it gives me a rediculous amount of volume and just the right amount of length. My eyelashes are very long, but tin and I don't have all that many. This actually makes my long lashes a little bit longer and awesome volume. To me, this is a step above the Falsies, expect The Falsies gives more length. Best of all, no clumping!! I recommend this in waterproof though because the original smudges.
If you need more length then volume, go with the Falsies.
If you need more volume than length, go with One by One.
EDIT: As for June 2013, this is no longer my holy grail, so sad to say. And I'm not sure what happened. I loved this more than anything, then tried a whole bunch for a while, was missing it, went back to it and now it's just... natural. Still basically clump free, very smooth, a little more on the voluminous side, but there was just no wow to it like I had before. Maybe I've matured a little into the makeup department or I'm looking for more now? PS-then I was 18, now I'm almost turning 21.
This is a great mascara if you have short lashes that don't curl. I prefer the the falsies over this in waterproof because I need thickness more than length. It is great for definition though. It holds my curl after I curl my stubborn straight downward pointing lashes! =)