Really solid clone - it looks and feels really similar to the original especially with regard to the enemy movement through Z space. It's nice how enemies really do move on their own and you can actually lose track of them like good enemies should do - I think sometimes they do reach areas where they are impossible to hit, but most of the time they move really well.
It is a little slow and I wish the max speed was a little faster, and as such the trees are almost a nonfactor since they are easy to dodge at such low speeds. I can't tell if speed increases each level, but having variety between the levels to ramp up difficulty makes it feel authentic.