Fun, but it did get old.
I agree about the slow spots when the screen is mostly clear. I would suggest either making the blocks come faster when there are less blocks; or letting the user manually add a line, either by clicking a button or perhaps by pressing the spacebar (or both).
I also found it a bit easy, meaning I got bored of playing before I ever lost, thoguh there were a few close calls. If you could think of a few more special blocks to intoduce as it when along, that would help keep things interesting too.
All-in-all a fun game though. It's always nice to see a new spin on a classic idea.