I'm not an expert but I can give you some info. I have a 4 way 15" IB.
1. Very few have reported any more noise in the surrounding rooms or outside the house than when they had a traditional box sub. Me included, no extra noise anywhere.
2. The sub is hidden and takes up zero space in the room.
3. Mine plays flat to 10Hz, has more output than my box could muster (AV15
PR'd), actually has more tactile feel than any sub I've ever heard/experienced. My entire house shakes between 10Hz and 20Hz sweeps. I also get the chest pounding sensation when appropriate.
4. The only down side I can think of is the absolute need to EQ the sub. With the EQ you can make it behave any way you want.
5. More drivers, more efficiency = fewer watts.
6. The cool factor is very high. Especially when nobody can figure out where the bass is coming from.
7. With multiple drivers playing in an IB you get much less distortion, the bass isn't colored, it integrates with my mains quite well.
8. Placing the IB between the mains is much easier than placing a bulky box sub between your mains unless you have a dedicated space for listening.
9. HUGE WAF if you can get past the hole cutting
With EQ my sub hits 10Hz without loss of output, I can't measure down to 5Hz so I can only assume that it has pretty decent output there but probably drops of a little. I'll be spending up to $1,500 for a 4 way IB once I have my final drivers. I could have one that is quite adequate for a 3,200 cubic foot room for about $700 but I want high excursion, not moderate so I'll probably be going the more expensive route.
For more info you must check this IB FAQ out:
http://home.comcast.net/~infinitelybaffled/