We recently purchased 2 Cosco Scenera convertible car sets for a trip (4 flights on Delta, total of ~20 hours in the air in coach). I cannot recommend the Scenera for travel, for the following reasons:
1) Wal-Mart, which is really the only place to get it, has a shipping issue. We had to order 6 carseats in order to get 2 that were not completely opened or crushed upon arrival.
2) The carseats have virtually no padding, which can't be comfortable for an infant/toddler on a long flight.
3) The attachment mechanisms are very poorly designed, likely a function of the low cost. While less of an issue on a plane, this caused significant problems at our destinations. The seats took at least 20 minutes each to install, and still were not fully secure. Moreover, due to the design of the front foot (see below), when installed rear facing in all but a large car you cannot use the Latch attachments because the foot will block the anchor points.
4) Very poor front foot design for rear facing. As noted elsewhere, when installed rear facing you need to use a rolled up towel under the front (now rear) foot. Unfortunately, no matter what we did we could not get a secure install. Additionally, while the seat is both very light and narrow (both good for an airplane), when rear-facing the seat is actually deeper than our large Britax Marathon. This is due to the front foot design which sticks out from the front of the seat. In fact, the Cosco seat actually fit WORSE front to rear than did the Britax - a particularly big issue when sitting in coach.