Jr. - I have played them, didn't mention them in my review for Gspot, mainly because the GBA game doesn't quite play the same. There's a bit more platforming than the arcade Slugs, and yes, the game has a health bar, but I wouldn't go so far as to call it analogous to the NGPC games or to say it's easy.
Even with all the armor and weapon card upgrades, and unlimited continues, level 5 is still one helluva pain to finish up.
Frankly, I don't think any of the additions take away from the formula -- it's still easier to die than it is to live. The first few levels are cake once you learn them, but 4 and 5 are setup to be longer and tougher so as to mitigate most benefits gained by the goodies and what-have-you. Skill is still the main factor for beating the whole thing.