Depends on how you look at it.
PRE-2007 lotto, the expectation was Greg Oden and a dynasty. This is a massive disappointment when held to those expectations.
POST-2007 lotto, the expectation was Yi Jianlian with the #5 pick... just total and utter hopeless despair. I almost quit being a basketball fan I was so completely depressed over it.
POST-2007 draft, the belief was that our GM was out of his freakin mind. Trading the #5 pick for Ray Allen in order to stay relevant for a couple years... a stop-the-bleeding trade to allow us to tread water and be a playoff also-ran. I was disgusted. Terrible, terrible, trade. I was angry.
Then the other shoe dropped and KG came to town. Expectation for many (including Bill Russell), was 2-3 championships. I still felt winning 1 was unlikely. Winning the 2008 title was so completely improbable and amazing to me that I don't even know how to put it into words. I've lived most of my life away from Boston as a hardcore Celtic fan. Being that I was born in 82, all I was use to was failure, failure and failure. Nobody in this state is a fan of the Celtics. I felt like a leper for even liking them. Winning the 2008 title was the greatest sports moment of my life. Massive unexpected and unbelievable success that I'll always remember.
With that said... I'll repeat what I've said in a few threads already. Revisionist history will see this Celtic team as a short-lived contender that only lasted for about 3.5 years (from the moment KG came to the moment Perk was traded), made 2 final appearances and won 1 title. That puts this team below the 00's Pistons (who were relevant for 9 years, made the finals twice, won a title and participated in the ECF 6 times). History will also mostly look at us as a footnote in the larger Kobe Bryant story. We're the "hump" he had to get over on his own. Winning 3 titles with Shaq, making the finals on his "own" (poor Pau), only to get stopped by the villainous Frankenstein Celtics in 08. He comes back and wins a title in 09... and then in 2010 vanquishes the Celtics to win his 5th title and repeat as undisputed Champion. Sad, but true. We're a blip in the era dominated by Duncan/Shaq/Kobe... and right before the LeBron/Durant/etc era. I can see how someone could consider THAT to be a failure.
It's all about context, I guess.