Any class with access to UMD, stealth, detection, OR support spells is guaranteed to be quite strong. With all of those together, Beguiler can be predicted to shine very brightly. I've written so much here that I've used Spoilers to compartmentalize it:
As Casters:
In dungeons, a Beguiler could situationally replace another caster; Greater Mage Armor, Haste, Imp Invis, See Invis, Mind Blank, and Freedom of Movement cover just about all the bare essentials. For non-undead dungeons, a Beguiler has access to the Mind Blank + Bewildering Cloud (or Confusion, or Deep Slumber, etc etc) technique, which can more than make up for lacking Stoneskin and Shield. A party that already has a Cleric, Paladin, or Ranger (classes that would not normally be able to sustain a party's buff needs alone) could reach near-optimal performance with the addition of a Beguiler.
As Skill Monkeys:
This is in addition to the Beguiler's ability to serve as a Rogue skill replacer. With Class Skill access to Open Lock, Search, and Disable Trap, there's no content that a Rogue can do that a Beguiler can't. Traps with DCs above a certain point can only be Disarmed by characters with Rogue levels, but as I understand it, Retrieve Trap functions normally for non-Rogues on these traps. Retrieve Trap carries a +10 to the Disable DC, but a Beguiler should have between +4 and +10 to this skill from Intelligence ability score, and gear could easily make up the difference if one remains. Also worth noting that a Beguiler could save a handful of skillpoints by not investing in Open Lock as much as a Rogue might, since a Beguiler can self-cast a high level Knock.
In PvP:
The Beguiler will likely be the strongest class for any PvP that does not begin with actual mechanical combat, and does not truly sacrifice the ability to gank. With Stealth and Detection, and cross-classed Set Trap sufficient to use the higher damage traps in the module, Beguilers are as prepared for espionage as Rogues or Bards-- moreso, even, because a Beguiler's self-cast Knock, True Seeing, and Glibness will function better than a Rogue's scroll-cast spells, and where Bards are restricted to Spot (which can't pierce Disguise), Beguilers have it.
Most significantly to me, on top of being able to reach the highest echelons of stealth, roguery, and disguise, and significant support magic, a Beguiler retains the most powerful survivability spells in the game-- Freedom of Movement, Ethereal Jaunt, Premonition, and Time Stop. With five level 9 spell casts per rest at level 20, and spontaneous access to their spells, it seems almost impossible to do anything about a Beguiler who intends to go where/do as they wish to do. Druids would be the hard counter to a sneaky Beguiler, because they could conceivably Spot them and break their Disguise, but aside from calling the Beguiler's place of business to file a formal complaint against their conduct, their options for any sort of mechanical aggression are limited in the extreme. A Beguiler could be expected to have long-lasting self-cast AC and immunity buffs, and would retain the ability to Time Stop up to five times consecutively without the need to premeditate their spell choices as a sneaky Pale Master might. Their lack of Discipline and weak physical saving throws are their predominant weaknesses, and these are mitigated by very high Armor Class and access to Spellcraft+Immunity-granting spells, respectively.
They
will struggle to kill things on their own unless they invest in gun feats, Set Trap, and damage-dealing spell scrolls, however.