Yes, it is schlocky and yes it is prime fodder for the Sci-fi Channel's "Mystery Science Theater 3000" (MST3K), but it is also a decent way to spend a couple hours without MST3K (the truth is that MST3K annoys me unless I've already seen a film once without it).
Bug begins like a disaster film, quickly turns into a monster movie, then evolves into a mad scientist tale - which turns out to be the bulk of the movie and which is in many ways the best part about it. In ways like a pedestrian version of Phantoms (though it predates the film), Bug is basically about a previously unknown species of underground roach that can start fires, apparently by rubbing their legs together like a cricket with legs made out of dry twigs. The roaches emerge after an earthquake hits a small town, and they quickly cause chaos - getting inside a pickup truck for instance and causing it first to stall, then explode while the occupants are still inside. The later "mad scientist" motif emerges when a local biology professor becomes obsessed with the creatures.
The Steinbeckian California dustbowl setting works well, and makes the first reel stylistic and eerie. Once the biology professor goes into mad scientist mode, Bug becomes appropriately claustrophobic and creates that icky sense of paranoia that should be present when you worry about bugs being everywhere you turn. Of course, some people might claim that relying on roaches is the easiest way to establish this. However, Bradford Dillman's performance as the professor gradually losing his grip on sanity is good, and this part of the film is tight, focused and scary.
The primary difficulty throughout the movie, however, is that it is difficult to keep this material from becoming silly. You could focus on the goofiness and try to watch it as a comedy, but it works too well as a horror film too much of the time for it to be successful camp. It is preferable to watch it as a straight horror film and try to indulge the silly bits. While there is not a lot of gore, there are horrific deaths, again made all the more effective because it isn't hard to imagine bugs crawling on you - it is one of our most primal fears and while you're watching Bug it causes you to look into your popcorn more than usual.
The score, an early 70's example of synthesizer composition (it reminded me of Morton Subotnick), while sometimes effective, is unfortunately dated in a way that makes it comic at times, also.
Bug provides a good time for anyone who can "give themselves over" to a film more than the average bear. Monster and horror film buffs who have a penchant for stylistic 70s films should also check it out if they haven't already seen it. Others should use caution.