Like all installments in the Blackdirge's Bargain Bestiary series, "Creepy Critters: Baleful Bugs" provides DMs (of D&D 4e) with a handful of well-conceived and well-crafted creatures. Players in my home campaign should not be surprised if they find themselves confronted by one or more of these creatures in the near future.
The stat blocks for these creatures seem entirely appropriate and innovative, although the puppeteer spider's webbed marionette power, effect 1, violates (intentionally, I presume) a well-established principle in D&D 4e regarding forced movement, and I probably wouldn't use it as written. The creatures range from level 3 to level 12, so you could find something here to challenge groups from the low heroic to the low paragon tiers.
Even though each creature is presented on one page with an illustration and a full stat block, enough flavor or lore text accompanies the crunch to help DMs incorporate the new creatures smoothly into a D&D 4e setting. I could wish, however, for less repetition in the flavor text. For example, one result in the scourgetail scorpion lore table reveals that "troglodytes actually enjoy [the] vinegary stench" given off by this creature, and the short paragraph under the "Encounter Groups" heading repeats this information in almost exactly the same words. Having been told this in the lore section, readers don't need to hear it again in the encounter groups section. Most of the creature writeups in "Baleful Bugs" exhibit this sort of repetition in the "fluff," and readers may wish those precious sentences had been used to convey new information about the creatures rather than to repeat information already given.