In my opinion, proceed with caution. Buy this only at $20 or lower. As the majority of Spongebob fans like myself believe, the overall quality of humor dropped relatively significantly after the Spongebob movie, starting with Season 4. For newcomers to Spongebob, PLEASE do not start here. I highly recommend that everyone reading this consider buying the Spongebob First 100 Episodes DVD box set, as it contains everything from Seasons 1-5.
Seasons 1-3 are the best of the show. They have the best humor, best overall pacing, and have great potential for multiple viewings. Practically every popular quotation from the series comes from these 3 seasons.
Season 4 is okay, but once Hillenburg left, the show lost a little creativity and humor. But overall, most of the episodes have at least something to offer, with only 2 or 3 episodes being notably bad.
Season 5 gets more iffy. High quality episodes are few and far between, with a lot of OK episodes. However, this season has more terrible episodes than Season 4, and is overall a step in the wrong direction. This is part of the reason I recommend the First 100 Episodes box set, as it has all of Season 5, and if you don't like Season 5, there is NO WAY you will like Season 6.
As for Season 6 itself, for those of you who read this far, Season 6 has, by far, the highest number of BORING episodes: not episodes that do anything heinous, but just plain boring. This comprises about half of the season. One third of the episodes in this season are VERY BAD, as in "You will regret your life decisions" bad. The remaining sixth are the highlights, the best of the season, and they are about as good as a decent Season 4 episode. Take that for what you will.
I bought this because I want to have all the Spongebob episodes, excluding Season 9 (Amazon has Seasons 7 and 8 on Prime Instant Video). I got it mainly to compare Spongebob from earlier seasons to later seasons. I would not buy it for any other reasons, and again, not any higher than $20.