The monocoque style of frame was popular in the mid to late 1990's, especially as a time trial frameset. Advantages are largely aerodyanmic - weight and stiffness was not noticably better or worse than the current carbon frames that sport a diamond tube design. Only figures I have are from the Trek Y-Foil, which was reported to have 30% less wind resistance than a standard diamond tube frame (but since the rider generates most of the wind resistance, it isn't that much faster) Some mono framed bikes also mounted the seat on a beam of CF - referred to as beam frames, which soak up some road vibration and are easier on the rider's behind.
Mono/beam frames went out of style when the UCI banned all bikes that did not have a seat tube in 2000. The actual wording of the ruling was weird - those frames represented an 'unfair advantage over developing nations', the truth seems to be that they just didn't look like bikes.
Three other mono/beam style frames that were made in the mid to late 90's:
Trek Y-Foil (see my avatar)
Zipp 2001 (actually made in 97-99)
Lotus 110 (full monocoque, very rare, designed by Lotus UK of auto fame, Chris Boardman set the hour record on one of these)
You can still find the Foil and Zipp framesets on ebay, usually $500-600 for a bare frame. The Lotus, sad to say, is not to be found anywhere - I've been looking for two years. (I'm an Esprit owner, would love to have their cycle, too)
One interesting side effect of a mono framed bike - they're noisy. The large center section acts like a guitar body, and amplifies any sound made. When I'm riding behind someone on my Foil, they say they can hear every gear change.