I don't agree spells should give a +5 skill bonus. That is pretty useless to casters (who generally lack skills). This makes a gish happy, but few others. Instead, I think spells should do one of two things (or a combination of the two).
Have a specific game-value of it's own. Invisibility could give a Stealth skill rating of +10 0r +1 per level, irrespective of what you started with. This makes spellcasters competitive but not overwhelming. The knock spell works like this.
Give new options. Invisibility gives the option of hiding in plain sight, flight, dimension door, et at - they all give you new options simply not available through skills.
4E's invisibility was of the second kind; it let you hide in plain sight, but gave NO bonus to Stealth, and you still needed a Stealth stunt for people to lose track of you. The result was that mages really couldn't become invisible at all.
