I like what you've done. Quick game, but challenging, and even a little painful :P. The upgrades really do make a difference, giving the player a sense of progress, and you really don't have to grind much in order to continuously afford them.
I have one major gripe. Do not, repeat do NOT, allow the clock to start until the player hits a button or otherwise indicates that he or she is ready. The fact that the screen is huge and I often spent the first few seconds setting it to a good place always hurt. The amount of times I lost by half a second is staggering, mostly due to the fact that the timer started running down before I was prepared to start mashing.