I personally am very fond of it and think that it is one of the best pClasses around from both a design and a playability standpoint. It is a powerful pClass, but it has stiff entry preReqs. If you want a Mage-Thief type character there is no better option.
It is on the high-end of pClass power, but it doesnt go completely over the top because most of its abilities are X-per-day; usually this is a bad thing, but thanx to the generous class features and spell progression it works for the Arcane Trickster.
One of the PCs in the campaign I run is an Arcane Trickster (Sor/Rog) and manages to pull thier weight without being too powerful, and a PC I play in another campaign is ramping up to it in a few more levels (Wiz8/Rog2/Barb1/with 2 levels in another pClass that grants +1 spellcasting); basically I need to take another level of Barb to avoid XP penalties and then another level of Rogue before getting into Arcane Trickster for my last 5 levels of progression.
BTW, yes my PC is widely scattered. He is basically an experimental character: he is the do-everything-possible character; he has the Tracking feat, Wfinesse & focus Short Sword, the Leadership feat twice (1 cohort is a dual wielding Fighter/Ranger/Barbarian maniac and the other is a Rogue/Cleric of Trithereon), and his Wiz spells (he's a Transmuter) are primarily buff spells---taken altogehter with his cohorts he's like a 1 man army and a very nasty piece of work. His 15th level feat will be Leadership again most likely, and I will probably take a Sorceror/Barb/some pClass or a Bard just to cover all the class bases. We just finished playing the original ToEE (converted and altered to fit 3e), and will possibly push on to do the Scourge of the Slave Lords after I DM my homebrewed Greyhawk campaign for awhile.