It's been said that good stories entertain and great stories surprise. By that rationale, Invincible is great.
Volume 1, Family Matters is a fun, lighthearted and entertaining story, and well worth your time and money; however, I'm going to recommend that you start with Volume 2, because it is surprising, and therefore great. It is a better example of what Invincible is all about: a well-written series with plot twists that you never see coming, but are still perfectly logical and plausible (well, as plausible as superhero stories get, anyway). The mood of the series can and will shift on a dime, going from funny to heartwarming to infuriating to depressing over the space of a few issues.
People who really want to jump into Invincible with both feet should pick up The Ultimate Collection. It collects the first 13 issues of the series (or the first 3 paperbacks). But if you want a small sample of a great comic, then start with Volume 2.
Recommended for: fans of Buffy, Smallville, the Spider-Man films, and/or John Hughes movies.
Not recommended for: people who prefer a more consitent tone to their stories.