We got one a few years ago for our amazon. Cannot recommend them enough! The pak-o-bird really opened up Kiwi's world to be able to go so many more places with us, which he loves. Much more convenient than a harness for excursions IMO. It puts a barrier between curious strangers and the bird (making the bird feel safer and prevents any potential bites). Also provides a barrier between your bird and any over curious dogs if your at a park or hawks if you have them in your area. It has easily removable dishes and holders for day or overnight trips/traveling where you want to feed your bird. It keeps your bird from pooping on you or holding it. It can be buckled into a car seat, but is still soft so I think in the event of an accident a bird hitting a soft front of a carrier would be safer than being loose or in a hard carrier (and it is too light to trigger an airbag, or at least in older cars like mine there has to be a certain weight in the seat for the airbag to deploy). I'm not sure about all areas, but Portland public transit has never taken issue with me bringing Kiwi on the bus or train in the carrier. It fits pretty well in the child seat part of a shopping cart if your bird runs errands with you, or just wearing as a backpack is pretty convenient too. Also kind of nice if your hanging out outside and don't want to keep a particularly close eye on your bird, but still allows them to safely be out with you zipped in. Some people even rig them up to fit in a stroller, though I personally kinda draw the line of pushing an animal in a stroller. I get enough stares as is having a bird in a backpack (and asked quite frequently if he's a "real" bird lol).
Only "con" I've found is the larger sizes are, well, large. We have a medium and it pretty much starts at my neckline and ends at the small of my back, like a very large backpack. But it's not heavy, just bulky (to give the bird enough room to be comfortable).