My girlfriend just got a used 2006 Birdy Anthrocite, which is a monocoque folder available in the US. I haven't seen a review of this bike here, so I thought I'd give it a shot.
First, I thought I might inherit the bike. It has a sport stem that puts you quite far forward and was supplied 170mm cranks. I was disappointed to find that my girlfriend wouldn't let me touch it, despite the fact that my Birdy Yellow is 165mm cranks and an upright stem, so it fits her properly!
The aesthetics of the bike are a matter of opinion, but I think it's a beauty. It seems to be oddly equipped, with the Capreo groupo, ergo grips (about 250gm relative to 10gm for foam grips), and a lot of complex design intensive bits about it. As a result, it's very easy to take weight off of the bike. But it leaves you wondering why you are paying buckets of cash for the standard set up.
On first ride, the bike is *amazing.* This is part of the reason I don't get to ride it. The bike is stiff and doesn't absorb one's efforts, but is smooth as silk on rough roads. The engineering is unprecedented. It is a very, very fast bike and a pleasure to ride. I don't even think the 250gm grips are coming off because they are so pleasant to hold.
The monocoque frame is very impressive. It's stiff and light, and the cable routing is very well thought out, running through the frame, and with extra cable ports in the event that you wish to upgrade to 2X9 speeds. The bike is completely silent, which is not unusual for most bikes, but is rare for Birdies, which are usually soundboxes.
The stem is stiff and very well engineered. It took watching the R and M video on the stem build to figure out how to adjust the tightness, though!
We went on a 200 mile tour of the C and O towpath from Cumberland, MD to Washington DC in 3 days (60mi-70mi-60mi on road, and about 10 off road). This path is packed dirt, mud, and gravel. It is a bit dodgy in parts, but generally well maintained. We had on road and off road riding. I can only report what my girlfriend noted, which is that it was the most comfortable bike she has ever ridden, much more so than the full size decent mountain bike we used in Thailand. We were so caked in mud after the first day that you could barely see the frame.
There is no hands-off shimmy on this bike, and, did I mention that it is fast? It is fast.
For those without cars, this trip is difficult to make because Amtrak, the US train system, does not allow bikes on board for small town stops for which there is no conductor to unload luggage. Folders can get around this problem, but you would be hard pressed to ride the trail on a bike with a frame hinge and no suspension, since the trail is quite rough. The Birdy got us around these problems by offering no hinged full suspension rides.
The bikes did not fit easily in the overhead luggage, but did fit nicely in the suitcase holds. I think I could have fit a Brompton up there and had the comfort of seeing the bike.
For those of you unfamiliar with the Birdy, it can be summarized as a bike that essentially has the same ride characteristics as the Moulton, but that actually folds to a small size (Bigger than a Brompton, smaller than a 20" Dahon.) It has no frame hinge. Instead, the wheels fold under the bike in the form of a pivot. An elastomer is wedged into the contact points between the frame and the rear triangle or forks, allowing for moderate suspension. The degree of suspension is adjustable, but the very light suspension cannot tollerate heavey riders. The bike can hold a 6'5" person and 250 pounds, including rider and luggage.
The major downside to the bike is that it is fussy to fold. It can be folded in under 10seconds, but this takes a lot of practice. When (not if) the bike gets jostled around carrying it, the chain can fall off. When this happens, it requires a good deal of messing about trying to get it unraveled. And that's as shipped. If the rider tries to modify the gearing with a new chainring, it can make the chain even more likely to fall off.
The other major downsde is that it is an expensive bike. The pressed monocoque frame and complex engineering features render the bike difficult to make. Therefore, the price tag is $1700 new (about $1450 online).
One minor problem with the bike is that, while it uses standard components, it is put together in a strange way. A sophisticated bike mechanic can deal with things like the upside down headset, but most mechanics will mess something up when trying to work on the bike. Finally, there are few dealers in the US. Unless you are fortunate enought to live in the UK, Japan, Germany, or Australia, it can be difficult to get ahold of a dealer.
Still, there is no other bike I would want to ride, big or small, on a long tour. I think it would make a fine racing bike as well. While a Dahon Mu will get you into a cafe or most buisnesses as easily as the smaller Birdy, it does not offer the same stiff riding characteristics or versatility (you can't take it off road, for instance). Thus, with the possible exception of the Go Bike reviewed recently, there is nothing in the category of small performance folder. I can see why there are so many fanatics out there. I've become one of them!