I don't think it flopped because I bet it's sales are more or less what Sony expected. The Go is an experiment, a companion model to the current PSP aimed at catching the early adopters for a totally digital model.
The Go cannot be compared to the Slim, a new model replacing the old, nor even the DSi (although I agree with Darth that seems the closest) because the DSi is also more of a new model than a total change of direction targeting a minority.
In the end the only way I see something as a flop is if it fails in achieving the goals set by it's company. Everything I've seen from Sony around the Go indicates to me it's performing as expected.
The issue I believe is that launching so close to Slim, everyone thinks its supposed to drive a big boost in sales in the same manner. It's not. It's aimed at a small number of early adopters (something it's price makes all too clear) whereas the Slim pricing makes it all to clear it is aimed at a larger majority than the PS3 models it is replacing.
I like the basic design of the Go but have no desire to switch, having a PSP with essentially the same features, a bigger screen plus access to both digital and physical media. But I can see that it is not the latest PSP model (in that it does not attempt to improve on the current PSP) nor is it meant to boost PSP sales (as explained already).