I'd say fun game, nice upgrading and weapon variety.
As for if the game is hard or not, as I see a lot of posts debating about it, I would say,
depends on how you play. Jumping and avoiding in the right time can get you far enough,
and right upgrades as well, and for most part you can access store at any time to just buy
some health packs.
I started with exp, health regeneration and health perks and
after that, all others and your set. It takes about 25 waves or 30 to complete the game, of
course depending on your upgrades and how often you fix the beacon, but after wave 45 it
will get very hard even with all weapons and perks, but then again not much of a use of going
any further than that.
Only thing that was bit annoying was changing weapons, going into back pack all the time or
scrolling though all weapons takes to much time. Simple primary and secondary weapon could
do the trick.
Other than that, nice game, I enjoyed this game.