Technically the spell never says that it isn't resisted, so based on that, we would go back to the rules for resisting Illusions, which says: "If the spell is not completely resisted, the target is fully affected by the illusion." So that means spells are all or nothing.
The text of the Camouflage spells(s) is that the bonus given by the spell is +1 per hit rolled on the Spellcasting Test. So if a target fails to resist the spell, then the full +1 per hit applies. This would be the strictest reading of the spell and how Illusions work.
Example: Spellcasting 5 hits. People attempting to view target of spell make resistance tests, if they succeed to get at least 5 hits or more to resist, the spell has no effect. Otherwise, the target gets to roll +5 dice to avoid being seen.
Based on the wording of the spell, it appears that the intent was for it to be a sort of "Aid Sneaking" spell, you cast it, and hits on the Spellcasting give the target a bonus dice on their sneaking tests. It is possible that the author of the spell didn't know or didn't remember the all-or-nothing nature of Illusion spells in Shadowrun.
Personally, I agree that the spell is poorly created. As is, even if the spell is 100% effective, the target can still be spotted if the viewer makes a good Perception test (since all it does is give bonus dice). I would save myself the extra dice rolls and let the spell just not bother with resistance rolls.
Extra Info: There is one other illusion spell that I know of that utilizes hits and not net hits from the Spellcasting Test: Hush/Silence. Now, the trick there is that it uses the hits on the spellcasting test for only part of the spell's effect. The spell itself is still all-or-nothing for whether or not you can hear while within the area of effect, but it also imposes a penalty to sonic attacks (or powers) that go in or out of the area.