I think you mean Unseen Seer in Complete Mage. Its a great PrC.
Typical builds involve either:
Rogue1/Wizard4/UnseenSeer10
or
Spellthief1/Wizard4/UnseenSeer10
Thats only 15 levels, the last 5 levels tend to be Arcane Trickster, Spellwarped Sniper (CScoundrel), or even a touch of Archmage is popular (for reach spell), although Archmage is VERY feat demanding.
I've also seen Rogue1/Wizard4/USS2/ArcaneTrickster10, but more levels of USS tend to be common just because USS has actual class features which are decent.
Unseen Seer penalizes your caster level for non-divinations. You can recoup this with the Practiced Spellcaster feat or the Master Spellthief feat (if you took the ST level). Since you apply things in the order most beneficial to you, you can apply the penalty first, the MST or Practiced Spellcaster to eliminate the penalty. Great spells to pick up with Unseen Seer's Advanced Learning feature are Hunter's Eye (Ranger2, PHBII) and spells like Divine Insight (CDivine) or Grave Strike (SpC) depending on your needs.
Great spells for an arcane rogue are anything that enable sneak attack (Greater Invis, Blinking, Grease, etc), and anything that lets you SA from range (Orb spells (SpC), ray spells, Cloud of Knives (PHBII), Whirling Blade (SpC).
Questions?