What's you opinion?

I assume you're talking "powered subs"?
I think, from an audiophile point of view, that subwoofers tend to "generally" get a bad wrap as being "low-fi" often. Really, it's my experience that, as individual components, A VAST MAJORITY OF SUBS(all shapes, sizes, and price points) are actually VERY VERY GOOD PERFORMERS POTENTIALLY, and are proportionately of a higher performance potential than MOST audio gear on the market! Let me clarify...
...What I find is that THE MAIN REASON for "POWERED SUBWOOFER'S" getting the rep they tend to is due more to POOR SET UP, CALIBRATION, LOCATION, AND INTEGRATION EXECUTION THAT USUALLY TAKES PLACE WITH THEM! Subwoofers are the single most placment critical components in the system largely, and people generally don't have a clue as to how to approach a subwoofer! Overall, especially since subs are mostly "powered", they have the potential for great control over Passive speakers and such, and will handle dynamics and transients a bit better on the whole.
I think people could get much better results sonically with subs if they knew more about how to set em up in the first place!

I agree with Foreverhifi2000 set up is EVERYTHING when it comes to a sub. Must use a spl meter with test tones at the minimum. I have a Velodyne F1800RII that I have modified and I couldn't live without it. In my opinion what makes a sub sound good is being as tight as posible with extension and the ability to integrate. Just my .02 John

OK, set up is critical. Where's the shortcoming? At the dealer, manufacturer or user end?

But what makes a good subwoofer?

What is your ideal subwoofer? Obviously it is one also with good instructions.

A good sub is one that when properly set up cannot be heard.

Why do audiophile's spend so much time and money on expensive amp stands? Why not just put your amp on top of your sub? The vibrations from the sub would have no effect on the sound would it? The answer must be no or they would not stick the amp inside a powerd sub. Also why do so many subs use cheap low-fi amps to power the bass when we spend thousands on our amps for the rest of our systems. Is the bass not as important as the mids and highs? Food for thought.