If you ignore space and cost factors, and just go on SQ they are great. By SQ I mean playback uncolored by the enclosure (and driver in some cases.)
If done right, a folded horn can have the least coloration. Tapped Horns and IBs follow, with everything else fighting it out below them.
The best way to describe an IBs sound is effortless, natural, and bottomless.
The rub with a horn is that they are bandwidth limited. If you have content playing below 20Hz you are really out of their element, unless it is a huge horn. They are also not naturally flat, and have built in "bass boost."
The problem for most people with IB sound is that it is pretty flat naturally. Most any other sub (unless we are talking about a ported sub designed to counter this, EBS...)
has a much greater degree of bass roll off as the frequency gets lower. What this means is that they are naturally hot in the 60-120Hz area, right where most of the thump in music is perceived at (that is also the area most coloration is located.)
What this means is that it seems like the IB (or LLT)
sub has less punch/thump than other set ups. It is just that the subs are not made with built in boost.
To get the sound most people want from there bass heavy music, you need to add in a lot of boost in the area mentioned above, or lower the bottom end same difference.
If you are big into bass heavy music/thump/punch, and your mains can't do it alone I would look to a horn, or more traditional set up.
The IB setup is the only sub I have seen that during a movie (The Incredibles, DVD) made someone watching it ignore the visuals totally, jump, and run to the window to see what exploded outside on reflex.
It was great. I had to rewind it to prove it was the movie.
That was at a friends with only a pair of Dayton IB15s, so it wasn't even up very loud -20/-15. (You should really use at least four, but he was really cheap. I had to add a 10Hz HP later to allow higher playback levels, and after I added it he ordered a 4-pack of AE IB15s.
Effortless, natural, and bottomless sums it up pretty nice.