Do ya feel lucky? Huh, do ya?
Well.. Theres two major glaring issues with this.
The first is luck. You are WAY too dependent on it. Either you get enemies, or you get items. And if all you get are enemies, you're sort of boned.
Secondly, and as a sort of continuation of problem one, the items change on you. When you unlock the Mommy and Daddy presents, suddenly thats all you'll find. Which makes things way more annoying than it should be since you go from having things more or less with decent speed/durability to once again, crappy weapons that break quickly. And now you ALSO have to deal with stronger enemies.
I think both can be easily fixed, by either making item presents a bit more obvious, or at least upping the drop rate of weapons. And perhaps make it so that weapon drops aren't completely switched over when you get to the next "stage"