This approach works well enough for relatively small amounts of objects. Once you start getting in to the millions (and significantly higher) then it begins to break down. Every "sync" operation has to start from scratch, comparing source and target (possibly through an index) on a file by file basis. There are definitely faster ways of doing it that scale to much larger object counts, but then they have their own drawbacks.

It's a shame the S3 Api doesn't let you order by modified date, or this would be trivial to do efficiently.