Good bug? Bad bug? Both or neither? Technically, they're not bugs. These are wasps, probably Pepsis thisbe, but I won't guarantee that.
I've been spotting of lot of them lately. This one, along with 2 others, was feeding on the Red Florence Fennel blossoms in the vegetable garden. They've been regular visitors in the garden and I've also seen them hanging out in the dry grasses on the hillside. I know what they are looking for on the hillside. And I'm pretty certain that they will find what they are looking for there because I've seen them. Tarantulas...
Here's an excerpt from the Wikipedia entry about Tarantual Hawks:
During the spiders's reproductive season male tarantulas are usually emaciated from ignoring food while searching for females. The tarantula hawks thus prefer female tarantulas and seek them in their burrows. They capture, sting, and paralyze the spider, then they either drag the spider back into her own burrow or transport their prey to a specially prepared nest where a single egg is laid on the spider’s body, and the entrance is covered. The wasp larva, upon hatching, begins to suck the juices from the still-living spider. After the larva grows a bit, it plunges into the spider's body and feeds voraciously, avoiding vital organs for as long as possible to keep it fresh. The adult wasp emerges from the nest to continue the life cycle. Tarantula wasps are "nectarivorous". The consumption of fermented fruit sometimes intoxicates them to the point that flight becomes difficult. While the wasps tend to be most active in daytime summer months, they tend to avoid the very highest temperatures. The male tarantula hawk does not hunt; instead, it feeds off the flowers of milkweeds, western soapberry trees, or mesquite trees. The male tarantula hawk has a behavior called "hill-topping", where he sits atop tall plants and watches for females that are ready to reproduce.
So, I suppose that if you have a Tarantula problem you might consider Pepsis thisbe and their relatives to be "good bugs".
On the other hand, Tarantula Hawk stings are reputed to be one of the most painful insect stings, possibly putting them in the "bad bug" category. More from Wikipedia:
The tarantula hawk is relatively docile and rarely stings without provocation. The sting, particularly of Pepsis formosa, is among the most painful of any insect, but the intense pain only lasts for 3 minutes. Commenting on his own experience, one researcher described the pain as "…immediate, excruciating pain that simply shuts down one's ability to do anything, except, perhaps, scream. Mental discipline simply does not work in these situations." In terms of scale, the wasp's sting is rated near the top of the Schmidt Sting Pain Index, second only to that of the bullet ant and is described by Schmidt as "blinding, fierce [and] shockingly electric". Because of their extremely large stingers, very few animals are able to eat them; one of the few animals that can is the roadrunner.
Since I don't have a problem with Tarantulas and do what I can to avoid getting stung, I tend to be neutral about the good vs. bad aspect of these wasps and regard them as simply beautiful and fascinating.