This was well done.  The difficulty curve felt good, making new levels harder without it feeling too overwhelming.  Some of the symbol combinations were definitely easier to find the difference than others, which is the nature of something of this nature.  

I liked that there were two different mode options.  I played more on the Classic mode over the 20 seconds one, but it was nice being able to pick which one I liked better.  I think it might have been a nice touch if each mode had its own high score tracking, over a single high score.

The timer sound was also a really nice touch.  I liked the fact that I didn't have to watch on the timer bar and could keep my eyes focused on looking at the symbols.

The minimalist style worked really well.  The sublet background gradient helped keep the background from being too flat and made it easier to focus on the symbols.  The white text on black was clear and easy to read.  The sound effects worked well, as did the lack of background music.  It added to the minimalist style and the fact that the focus was on testing your eyesight.

I think this was well made and you did a good job with it.