Persistent Spell: is it worth it?

[Jack Simth]

How powerful IS a persisted 1st or 2nd lvl spell, when compared to the 7th or 8th lvl spell (assuming no MetaMagic reduction or Divine Metamagic substitutions) it replaces?

Well, consider:
A Persistent Detect Magic (6th level spell slot) saves you the 500 xp for Permanency... every time it gets Dispelled successfully (Ditto for Read Magic, Comprehend Languages, See Invisibility, and Arcane Sight, at different XP values and spell slots).
A Persistent Shield (7th level spell slot) is +4 AC that doesn't interfere with a Monk's Belt (although if you're not using a Monk's Belt, then you're probably going to go with a Mithral Buckler with some bonuses)
If your DM permits Touch range spells to count as fixed range, then a Persistent Protection From Evil (7th level spell slot... you can also substitute any of the other Protection from Alignment spells...) makes you immune to the heavier-duty mind control, makes you mostly immune to Summoned Evil (and neutral) critters, and makes you immune to Magic Jar.
In the right campaign, a Persistent Detect Thoughts is crazy-useful for an 8th level spell slot (in others, it's basically useless - buyer beware).
A Persistent Mirror Image might not be worth the 8th level slot (or Displacement at 9th, if your DM permits Touch range spells to qualify)... but it does let you cast that very useful defensive spell WELL in advance of an actual encounter.

And those are just with Core spells.