I spent $275 used for a modified one of these. The one I have has about 6 inches shaved off of the top of the lid. It just barely fits under my computer table.
http://www.randallamplifiers.com/products/cabinets/isolation/index.asp
Basically it is a box with a 12" Celestion Vintage 30 (90watts and 8ohms I believe). The box is totally closed. The mic input leads to another cable which will attach to your mic on a built-in stand inside of the cabinet. I got the gooseneck attachment so I could get the mic really close to the speaker so I could get the brightness I require to keep my sound crisp.
The top has a latch and it keeps an amazing ammount of sound contained within the box. I can turn up my Triple Rectifier to about 3-3.5 which is actually pretty loud when plugged into the 4x12 cab. Playing live shows, I never turned it up above 4 or 5, even when my cabinet wasn't miced.
At that volume, within the Iso cab, it sounds like a medium level boom box listening to music. Not something you would want to do at 3 in the morning in an apartment building with someone living right under you, but not something you would be affraid to fire up at 8 or 9pm. I routinely record at 9-10pm and never have any complaints. The only thing they probably hear is some of the bass transmission from the low chunky notes. Other than that, it's really damn quiet.
Sound? Sound will depend heavily on what mic you use and how you place it. I got a Sennheiser e609 (I think thats the one) with the silver grille. A difference in 1-2 inches in placement from the speaker made a world of tone difference. It went from boxy and flat to having lots of definition and clarity. Mic placement is super important.
The box basically seems to get raw speaker tone. I might be wrong, but it doesn't seem to be colored by the box (or anything else) at all. The sound I hear in my headphones is what I get when I record... and the sound in the headphones sounds pretty damn good.
Also to be noted, I can easily crank my headphones to drown out the sound from the Iso cabinet without having it sound ear-damagingly loud through the phones. That's how quiet you can get the Iso.
I would reccomend it.
Demeter also makes a similar cabinet.