It's a little early to say whether or not Curiosity's discovery of complex organic molecules constitutes a let-down or not. I think whenever information is teased in this way to the public: 'oh, the rover found something, but we can't tell you what it is just yet,' there's bound to be a feedback loop of speculation, official silence, and then even more speculation. The first rumors of organic chemicals quickly became sigs of microbes became like actual Martian fossils, right there on Mars! Now we're back to chemicals that might be organic. So, that's something. Unless it's just contamination from Curiosity itself which is always a concern.
I am actually ambivalent about the whole question over life on Mars. If we find it, of course that would be an enormous discovery, mind-blowing really. But if we never find life, I think in someways that might be nearly as significant.
The discovery of life on Mars would suggest that life is in somewhat common in the universe. In our own solar system, two worlds with very different histories and geologic make-ups, would have both produced life. The odds would dramatically tilt in favor of other discoveries. The universe might become a little more full and also just a little more crowded. It would be very a very important discovery.
However, just for a moment, consider the alternative. Suppose life is not so common and Mars is but the first in a long future history of disappointments. What would that mean? Well, for one thing I could touch upon a piety I've heard more than once that it might help this species understand just how rare and precious this planet is. Maybe just maybe we wouldn't be in such a rush to pump out even more hydrocarbons if we had the suspicion that we are it. Alone in the universe. If there is no other life out there to pick us up if we ruin our home world maybe we should take care of things a little better.
But what I really think is that a Mars empty of life is what Kim Stanley Robinson proposed, an irresistible real estate opportunity. Nature abhors a vacuum and Mars is a lot of empty land. I guess that might sound a little crass but I've never had a problem with the notion that Earth is not the be all and end all of this species. The capability to leave this planet is a nearly unassailable moral imperative to do so.
Although judging from this article, it's an open question about who will be doing the exploring.