Generally speaking, some umpires suck, but very few suck as bad as fans think they do (exceptions include the "my zone" guys Midlo mentioned).
It is hard to call strikes accurately and consistently. Good umpires approach the job with awareness of one's perceptual limits, concentration and humility.
The catcher and the plate umpire have the best view of the three dimensions of the zone abd the whole flight path of the pitch.
Other observers--coaches, fans, base umpires--sometimes have more accurate views of one dimension than the plate umpire (e.g., the base coach faced by the batter can know that a pitch is above or below the zone, a fan directly behind the outside corner can sometimes know a pitch stayed outside the whole length of its path.
However, these observers can never know for sure that a pitch was a strike because the ones who can judge up-down can't judge in-out, and the ones who can judge in-out can't see when the up-down was right.
Most observers show little awareness of the depth of the zone.
Umpires look to see any part of the ball in any part of the zone.
Fans simply cannot see the brilliant top-of-ball/bottom-of-zone/front-of-zone and cb-nicks-outside-corner/bottom-of-zone pitches that look horrible to them.
The cues most fans and coaches use provide irrelevant information about where the ball was after it passed the zone.
Despite the perceptual limitations of the umpire's vantage point alluded to by Matt, there is no substitute for tracking the whole flight of the ball from release point to mitt with a stationary head. People who can't do that from where they stand should be circumspect about denouncing those who can.
Sometimes you can know we're wrong, but most of the time you can't.
And that would be true even if your perception weren't distorted by your desire to see the ball either in or out of the zone.