Leet
Even though you're doing pretty much the same thing the whole time the upgrades and new enemies keep it entertaining.
I think being able to heal while playing makes it a bit too easy though!
Lucky I got a new PC recently, cause it lags a bit even on this one! I can imagine people with crap PCs will complain about massive slow down in the later levels.
For improvements I must suggest some boss-type enemies, that have more interesting attack patterns and mass HP, and require a bit more strategy.