I really like the design of this carrier for allegiant airlines size requirements. I enjoy that the top expands so that the dog can turn around in the bag and pass the turnaround test if the check-in people make you do that. I have had them make me do that once…I have a 22 inch four wheel rolling, hard side, carry-on bag, and I can set this bag on top using the trolley sleeve, but it’s a little awkward and you really need to use the top straps to provide leverage by putting them over the handle on the rolling bag or you need to loop the long strap around the handle of the rolling bag to prevent it from tipping with the dog inside it. You will see what I mean when you start to use it. I do like the very firm base inside the bag.REMEMBER THAT IF YOU ARE TAKING A PET CARRIER ON A PLANE YOU STILL ONLY GET TWO CARRY ON BAGS (ONE IN THE OVERHEAD AND A SMALLER ONE UNDER THE SEAT) THIS IS A USA FEDERAL THING, NOT AN AIRLINE THING. SO THE BAG COUNTS AS ONE OF THE TWO EVEN THOUGH YOU PAID A PREMIUM TO TAKE IT ON. PREPARE TO CONSOLIDATE OR CHECK THE EXTRA BAG.Most airlines let you count it as your personal item because it goes under the seat, but you better check.Normally I want my dogs to go in and out the side of the bag. The end of the rectangle side that will face you when you are in the airplane (like they are going in and out of a dog kennel) but this bag has a really small panel on that side to let the dog in and out. I don’t really like that about it. I would prefer that that were a bigger panel, but I can see why it isn’t because the bag has metal reinforcement on the sides and I don’t think it’s possible to make that panel bigger. So, I have to put my dog in the top, which is OK,especially with the expanding top.The bag that I use on bigger planes does not have a top entry (sturdibag).My middle sized Boston terrier is about 15 pounds and she is kind of long but she fits in the bag just fine and she can stand up (without lifting her head completely) turn around and lay down comfortably. Under the seat with the top popped down, she can turn around if she crouches and scoots… but I pull her out from under the seat and check on her frequently so she can turn around and stretch if she wants. My 19 lb Boston doesn’t fit well enough for comfort. She can fit in the bag and turn around with the top popped up but she can’t stand up. She has to stay lying down or crouching. She would probably pass the check in people if they didn’t inspect her too closely, but I wouldn’t chance it. Plus it is not nice to her… at all. My tiny 10 lb Boston is no problem. She can run loops in there.Someone wrote that they did not like the carrier because it did not fit under business/first class seats. That is a common problem with all business class seats. They are quite low to the ground, but typically they are not going to kick you out of business or the plane if your carrier fits their standard size requirements for the coach seats. They will just allow you to hold it in your lap or something until the plane takes off and then you could put it on the ground by your feet until landing time. (Been there and done that with the much bigger but more squishy sturdibag). Depends on the flight attendants of course…. Yes, holding it in your lap is probably a big no no but what else are you going to do? You can’t put an animal in the overhead… dogs have died that way. So as long as it meets the airline size requirements, I am sure they will work with you in first class. Plus you can hold babies in your lap… and not pay for them…. Hmmmm.For Allegient, I highly recommend a middle seat or extra leg room seats. See my photos. The photo with the carrier in the middle seat is taken in the extra legroom seat section. The photo with my feet visible is taken in the extra (premium legroom) legroom seat section in the window seat and the seat where you can’t hardly see anything is a regular seat. You will have zero legroom if you sit in that seat with this carrier, Yet somehow it is still compliant. If you have to sit in a regular seat or you don’t pay extra, please choose a middle seat. You will be able to turn the carrier sideways as seen in the pictures and it will fit perfectly. [...]