KDE 4.0 was still somewhat immature when it was released, but I'd hesitate to call it a debacle. From what I remember, it was meant mainly for early adopters, so that issues could be discovered and resolved quickly. KDE 4.1 addressed many of these. KDE 4.2 and subsequent releases have provided an extremely robust desktop environment.

On the other hand, GNOME 3 was a debacle, without any doubt. KDE 4 resulted in nowhere near the level of backlash and response that GNOME 3 did. I mean, GNOME 3 was and is so bad that it spawned two rather significant and ongoing salvage attempts in MATE and Cinnamon, in addition to completely driving away a lot of existing users. We just didn't see anything close to that with KDE 4.