Even without considering the five levels lost to wizard, an Arcane Trickster only has 4+int skill points, rather than the 8+int of a true rogue. He also doesn't get Trapsense or Skill Mastery, both of which I would consider essential for the skillmonkey, and his new abilities don't really synergize well with his skills (ranged legerdemain is OK, but you can usually just walk up to the lock or pocket or whatever you're trying to pick).

If you want a mage/thief type character, go with a Spellthief (if your DM allows them).