Cowboy Bebop is a series that's great because of it having the best, most cool main character ever and one of the best endings ever. Up until the last two episodes, I was thinking of it as a 7-8 series that, although enjoyable, was too shallow to be rated as one of the best. I then got to the end, gave it a rating of 9/10 and paid £60 to buy the series on DVD.
I remember having a MSN conversation with the guy who recommended it to me originally after the episode where Spike and Vicious faced off for the first time. At that point, like when I was halfway through Gankutsuou, I wasn't thinking of it as one of the most amazing series in existence, in the case of Cowboy Bebop because of its episodic nature. It's one of those series you really need to watch to the end in order to judge it.
Samurai Champloo, which was made by the same people as Cowboy Bebop, is the same sort of thing. Until the last three episodes, I was going to give it around 7/10 - I ended up giving it a solid 8/10. Samurai Champloo was weaker than Cowboy Bebop because of its characters, yet the final three episodes made up for most of its failings.