Schmoe said:
On average, I like saves as follows:
1. Good saves - 75-80%
2. Poor saves - 40-50%
Those numbers feel about right to me, intuitively.
Just out of curiosity, let me show a few numbers, from a purely arbitrary case.
Let's assume a CR 9 monster (a vrock demon), against a party of 9th level adventurers.
At 9th level, your "good" save is +6, and your "bad" save is +3.
The DC for saving against a vrock's nastiest "save-needed" attack (Dance of Ruin) is a DC 18 Reflex. (And, that's probably an average difficulty, at worst, for threats at CR 9; a 9th level wizard or cleric hitting you with a 5th level spell is going to have a save DC of at least 18.)
To hit an 18 75% of the time (good save), the PC needs to be able to save on a 6 on the die. That's a +12 total modifier, so you'd need an extra +6 from either ability scores or magic items. Not outside the realm of possibility, esp. for those classes who have good Reflex saves (which will also tend to have good Dex scores).
To hit an 18 50% of the time (poor save), the PC needs to be able to save on an 11 on the die. That's a +7 total modifier, of which +3 is coming from your base save. Again, not infeasible, esp. if you have a positive modifier on your Dex.
Where the saves really get tricky is when you have the combo of (a) a poor save, and (b) a dump stat (Will save for fighters and rogues, I'm looking at you).