Really glad I chose this travel bag for my little dog (~4.5 kg): it fits perfectly under airline-seat rules (55 × 40 × 23 cm) and gives him enough space to lie down comfortably. The soft sides plus the rigid removable base combine flexibility with stability — I felt confident at check-in.
The mesh ventilation is a big plus, especially for the short European flight we took. Only minor point: because the bag is made to very strict size limits, taller dogs might find the height (23 cm) a bit restrictive for standing fully upright.
But for a 4.5 kg breed this was absolutely fine. Would recommend it to fellow German travellers who want a smooth cabin-flight experience with their small dog.