Crankypants, I really enjoy my siver birdy. It is light, folds small and is comfortable to ride. It seems to be expensive but it has full suspension and has easy height adjustment of handle bars and seat. Its components are of good quality so it probably is a case of getting what you pay for.

In Australia the distributor is very helpful and has a full range of parts and accessories.

As for tyre choice he has four types of tyres available:
Off road
Maxis kevlar 90 psi
Schwalbe Marathon Racer
Schwalbe Stelvio 120 psi

I have read recently on the birdybike forum that a Schwalbe Big Apple will be available soon, although it might not work with all models (fouling the derailier I believe). With the full suspension I think you can use high pressure narrow tyres.

As for noises apparrently this varies between models and components and there have been posts to deal with these, although it may just be part of this bikes design and magnified by the aluminium main tube resonating.

I have been careful maintaining brake pads as I have found metal embedded in the rubber scouring the rim.

The Australian birdy has a trigger rapid fire type of gear changer which I like and apart from this I have just added moulded bar ends. I have no experience with mudguards or carriers.