Personally i think the arcane trickster is fine. It has the hardest prereqs of any PrC i've seen in any wizards book yet. For this you get Full Casting Progression (which just about every wizard class in existance gets), increase to Sneak attack (which you sorely need at this point since you had to take 5 levels or mage or 6 of sorcerer to get into the class), 4 skill points/level which does little to help your lack of rogue skills from the 5/6 level of mage, and 2 moderately cool abilities. Is it better than a pure Rogue/Mage? of course it is, rogue/mage isn't a very good combo to begin with, but is a Loremaster better than a Mage, a Bladesinger better than a Fighter-mage, or a Order of the Bow Initiate better than a standard archer? Yes to all those as well. I've been playing in a campaign for 9 months now and one of my players is still trying to get into Arcane Trickster. He's spent alot of time with low hit points, not much magic power, bad rogue skills, bad BAB, and little Sneak Attack. He has focuses on aquiring spells that augment his rogueliness, which has helped quite a bit. So if you are going to make up a 15th level char from scratch Arcane trickster looks pretty good, if you are playing one from level 1 it's quite a different story.