It's a fairly solid class. It can handle the face role in the party, while also providing exploration and stealth abilities. As noted, it focuses on illusion and enchantment. While it has strong options for a wide variety of situations, against certain foes, the beguiler may have to fall back on UMD. It's a somewhat popular choice for combing with cleric, for a mystic theurge with a very strong suite of illusions and enchantments, or with a prepared caster like wizard, for a somewhat specialized ultimate magus.
As a single class, it's rather like a somewhat less melee capable bard with better casting and no music.
It doesn't synergize perfectly with arcane trickster, but AT functions as a decent alternative advancement at middle levels. EDIT: It generally does, however, require shenanigans to get mage hand, as that spell is inexplicably not on the beguiler spell list (open/close is).