Open theists believe that 1Sam 15 highlights the case of a man (the king of Israel himself) grieving the heart of God by refusing to obey Him (two incidents are recorded in 13:7-14 and 15:2-9).  This failure hurts God precisely because our Lord had hoped that Saul would be a faithful and obedient king, failing which God grievously repents of His decision (15:11) and then SEALS this change of mind by denying any more opportunities to Saul to win His favour (15:29).  The broken and regretful heart of God for having initially made Saul king is emphasized all the way to the very end (15:35).

15:11 and 15:35 are quite unambiguous as to the fact that God has changed His mind about Saul and genuinely regretted anointing him to be king.  This makes nonsense of the notion that God knew 'all along' that Saul would fail, and fits in very well with the conditional nature of God's promises to Israel and Saul in chapter 12 and 13.

Most insightful is Samuel's warning cum premonition to Saul after his first act of disobedience (in 13:7-9).  See 13:13, "You (Saul) have not kept the Lord's commands; if you had, He would have established your kingdom over Israel for all time".

God really intended to bless Saul greatly upon the condition that the king remained faithful and obedient (and in fact the first warning above could be seen as a divine attempt to urge Saul to do so, although it could equally be true that Saul had already 'sealed his fate' from that point on).  Yet after the second act of disobedience by Saul (see 15:4-9), it was clear to God that a man 'after His own heart' would have been a better choice of king and that anointing Saul was a regrettable decision given the guy's failures.

The declaration in 15:29 that God will not change His mind needs to be understood not as the primary statement of divine attributes under which vs.11 and 35 must be subordinated and interpreted (much less need it be seen as a contradiction in Scripture), but rather as the culmination of a series of rebukes aimed at Saul to finally hammer home the Lord's judgment upon him.

A look at the context strengthens the plausibility of this interpretation:

15:24-26, "(Saul said) 'I have sinned...I violated the Lord's instructions...Now I BEG YOU, forgive my sin and come back with me, so that I may worship the Lord.'  But Samuel said, 'You have rejected the word of the Lord, and the Lord has rejected you as king over Israel!'"
(Strike 1 for Saul)

15:27-29, "As Samuel turned to leave, Saul caught hold of the hem of his robe, and it tore.  Samuel said, 'The Lord has torn the kingdom of Israel from you today and has given it to one of your neighbours...He who is the Glory of Israel does not lie or change his mind; for he is not a man, that he should change his mind."
(Strike 2.  Nevertheless, Samuel still followed Saul back to Gilgal and God still accepted Saul's worship [see vs.31]; maybe this is why it needed to be affirmed again in vs.35 that God still regrets His decision to make Saul king as vs.31-34 may have given an opposite impression).

In my view, 15:29 does not and can not mean that God NEVER changes His mind or regrets His decisions.  The verse (like Num 23:19) makes God's refusal to change His mind SYNONYMOUS with God never lying i.e. it declares the STEADFASTNESS of God's actions in a specific situation for which He has a declared purpose.

God has pledged Himself to a set purpose, and His pledge contains perfect integrity. He is not like Man who has a tendency to say one thing while fully intending something else (an issue which classical theists really need to address, since they believe that God can declare plans whilst never intending at all to carry them out).  See one classicalist's objection to this from John 6:4-6.

And this is the message of the 'non-repentance' passages (only two, mind you).

The writer of 1Sam was most assuredly not explaining to Saul (and us) the fact that God's repentance does NOT include mind-changing; what in heavens for?!  The context is one of firm rebuke from a prophet to a disobedient king who knows he has sinned and is begging to retain his kingship.  It would make more sense to see Samuel's affirmation of divine non-repentance as applying to God's SPECIFIC judgment ("The Lord has torn your kingdom from you and given it to one of your neighbors") in the midst of Saul's REPEATED pleadings for another chance.  All this is entirely congruous with the front-and-back affirmations of divine repentance for having made an unfaithful man king over Israel.

I believe this interpretation of 1Sam 15 fits in very well with the rest of the Biblical portrait whereby God allows His people to influence His decisions (especially on judgment issues, like Jer18) yet also affirms the solid integrity of God.  God is truly in personal relationship with us, taking risks with His bestowment of special privileges, allowing us to affect Him even as He blesses us, seeking to woo our hearts towards His own, and withholding judgment when He sees changes in us for the better.

Such is a more Biblically-based, rich and dynamic understanding of the PERSON of God than one which employs philosophical reasoning to deny from the outset any change whatsoever in all aspects of God.

Now let's zoom in even closer on 15:29..."for God is not a man, that He should change His mind" - Does God's divinity preclude mind-changing?  (like some Reformed theologians claim this verse teaches?)