Anecdotal evidence and all that, but I've played two Beguilers in PbP games for a while now, and they've more-or-less yet to do something truly useful in combat. I think that between the two of them, they've downed one kobold mook (they are levels 3 and 10). However, they have participated in lots of fun side-intrigue when going in and around towns instead of out in the wilderness or a dungeon. This, however, depends on the willingness of the GM to throw you those kinds of things.
I've found that despite the deceptive combo of high Int and good skill points per level, I run out of skill points faster than for any other class, since the Beguiler pretty much requires Concentration and Bluff maxed, then I like to go for Cha and Int skills like Spellcraft, Knowledge Arcana, Diplomacy, etc, and the group expects me to have sneaking and trap-disarming skills (thanks to the Trapfinding ability, the Beguiler is often de facto the only one who can find the toughest traps, unless you also have a Rogue, in which case good luck with undead).
At low levels, the Beguiler's offensive power is stymied against a wide swathe of opponents. Undead, constructs, oozes, plants, etc are completely immune. Many other monster types have good Will saves and high HD for their CR. This leaves you with the option of making an underWhelming (pun intended) damaging attack with a Will Save on it or doing a save-or-out with a Will Save on it (Sleep, Colour Spray, etc), so unless the monster has enough HD to force your hand, the choice is clear. However, if you aren't playing that crazy gray elf in post 14 who somehow has enough point buy for 20 Int and adequate numbers in the other important Beguiler stats (which are generally everything but Strength and Wisdom), you often wind up with about 16 starting Int, and so either DC 14 spells or DC 15 if you're taking Spell Focus and delaying Improved Feint, which should usually be picked up ASAP to allow for Swift Cloaked Casting. Opponents other than low-HD-humanoid-with-Fighter/Rogue/Warrior-class-levels don't often fail DC 14 Will saves (in my case, just one kobold mook has failed so far), and the Beguiler doesn't even really have good combat buffs to start off, so you usually wind up making like a Wizard out of spells and firing crossbows.