So far, so good. Jur convinced me to try the Slime liner, and it seems to be a good trade off between weight and protection. Some say that they themselves can wear out a tube, causing a flat. But little wheels with high pressure tires hardly deform at all, so I doubt this will be a problem.

It sure would be fun to test the efficiency losses with a power meter. I hate getting dropped on fast group rides. (Of course, nothing will drop you faster than a flat.)

This is my 2 second summary after way over 10,000 miles:

Upside:
Spoke for spoke and pound for pound, I still think that the Birdy is very fast so long as you learn not to bob, stand, or pull. It is more aerodynamic than other bikes, and, for the well adapted rider, is probably a second only to the Moulton in terms of rolling efficiency. (On the other hand, road bikes are lighter and have fewer spokes, so the argument is moot; Moultons now routinely get beat, and only hold the speed record because they were run on a flat course). It's best use outside of urban multimode commuting, is in touring, where it can fit easily into a suitcase, can hold 35+Kg and still be folded to take inside guest houses. In the urban setting, it is by far the lightest bike, since you do not have to carry around a NY Kryptonite Chain.

Downside:
It is not as stable as full sized bikes at high speeds, and you can't take your hands off the bars (due to shimmy). You also have to buy tires (but not tubes) online if you don't have a dealer near you. If you are a stander or puller, forget it. It may be too responsive for some. There are more parts than on a normal bike, and those parts are over engineered. I've found that the steel used in some of the bolts rusts when exposed to sea water, so they need to be replaced with better steel or titanium...kind of a drag on such an expensive bike. Finally, on the older bikes, like this yellow, the seat should be pushed all the way forward. I had a seat post break on me. This isn't a problem with the newer posts. Finally, it doesn't roll on it's own when folded unless you get a fancy rear rack for it.