Just to resurrect the thread: I had a chance to ride a Birdy last weekend. I've always admired the Birdies from an engineering point of view, as they seem to be designed as a folding Moulton.
The ride was very smooth but also had a positive pedalling action. However, the position felt very upright, with a very short (effective) top tube. There may be some way around this, adding a stem or changing the settings.
The fold was not too small, nowhere near as compact as a Brompton. It basically folds in two and the handlebars fold down. It was light enough to carry, however.
Would I buy one? If I needed a folder I'd certainly look at one again.