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.

Biggest problem I see with most sub set-ups, right up there with improper placement, is trying to integrate a sub with small satellite speaker systems that have little or no bass below 80Hz. This forces people to push the subs up into the 100+Hz range where they really don't work as well. I started messing with biamped and passive subs back in the late 70's as LS3/5A's and KEF 101's and several other monitor speakers rose to popularity. The results were never completely satisfying. I finally gave up. Years later I read a quote that finally pointed me in the right direction. It roughly said that "It is difficult to integrate a sub with "main" speakers that have poor bass output". Recently, I have heard some really good main+sub set-ups where the mains were full range speakers with GOOD bass to about 45Hz; they just needed a little help in that last octave. Crossed over at about 60Hz, many of these set-ups sound quite good. (You still need correct placement.)

Well...if you think there are very few decent loudspeakers...try finding a musical sub...slim pickings...there is REL,Sunfire(much to hi enders dismay),M&K, hi end Velodyne...ect...

Subwoofer shouldn't sound like anything. Meaning if you hear it, it's probably set too high or wrong room placement.
HSU has a VTU-2 (?) about $500 or REL are both good.

In response to Gmueller's comments, I find that THE VAST MAJORITY of people setting up sub/sat systems have THINGS WAY WAY OFF in terms of "IDEAL"! I do however find that you can get excellent, to phenominal results with the right gear, properly set up...even if the mains/sat's only play down to roughly 80 hz! The key is proper coupling with the room at the critical "crossover point" for both sub and sat's, balanced/flat frequency response for sub and sat's(usually not so good, do to user error and inexperience), proper phase between sub and sat's(definitely at the crossover reigion), and proper level matching. if you can pull all that off(takes some experience) well, you'll have dynamite sonic potential(acoustics and set up for soundstage is the rest)!
Of course, some subs will play "tighter" or with more authority up into the higher registers of the bass than others, just as some subs are more "accurate" sounding than others. A powerful, yet not so accurate sub musically might keep your potential down. Quick, accurate, dynamic subs, that blend well from a crossover standpoint, and play higher up better than others, obviously have an advantage. Higher "Q" subs are going to generally sound "more controled" and accurate on the whole. It's always a compromise, especially at the lower price points.
Still, there's no reason that you can't get the same potential "world class" results from properly setting up a sub sat system, as you can with an integrated full range speaker system! Infact, you have more flexibility with the sub/sat system. Still, unless you know what you're doing, you can do "more things wrong", or less than ideal with a sub sat system as well...DOUBLE EDGED SWORD!
Still, I know what I'm doing, so I like that option for a lot of applications.
Hope this helps

I build mine.I find fraws in most designs.15-18 inch drivers in a sealed box.Mid Q design seem to work fine in the HT arena.Amps must be 800-1000 watts,with a real ampere output current in mind.Cabinets have to be at least two inches thick of MDF.Heavy on the bracing too.I tend to use two in my system, it takes the edge off of the other driver and always seems to give me the headroom I need.It needs to play clean up to 110-115db to get that,"I'm not even working feeling" when playing around 80-85db.Nothing is worse than a sub running out of steam at around 85-90db.I cross mine over at 65hz.That's where my sats. rolloff.Room modes suck.You have to get a parametric to keep out the bumps or all you will hear is the bumps.I have been building them for about thirty years now and seem to out grow them every five years.My neww ones will be using Adire drivers(tumults) and Adcom GFA 555mkII's in mono.Wish me luck.