Upload & Sell: Off
I was most favourably impressed by the "upright" perspective tool which is very handy for squaring up documents and product boxes that I photograph because a flat surface photographed at an angle to avoid reflections ends up looking right as if I had photographed it square-on. It's not perfect, and in some cases it won't work at all (especially if it sees text at different angles such as on the vertical and top faces of a box), but it usually helps a lot.
It would be far nicer if I could pinpoint the four corners of a photographed document and tell it to square it up with a ratio that matches the original document (e.g. for A4 or whatever). However, the tool is fully automated and does not allow this. Great if it works as you would hope but no good when it gets confused. You see, one of the problems is that it looks at the entire image and cannot be restricted to look at just the bit that you want it to work with.
Something like DxO Viewpoint may be better in some ways but it creates separate edited files whereas the upright tool does not have to.
Because I use a laptop with external drives for primary photo storage I am also interested in the new smart previews for when I want to go portable for more than just a few hours at a time, but I have not yet tried it. I'm hoping to have smart previews on an internal SSD and so be able to look at or edit any of my images while out and about without taking the external storage with me.
Importing a Lr 4.4 catalog is probably not reversible but that is not an issue because Lr makes a separate copy, leaving the older one intact. It does, however, mess up the previews or at least re-use them and so it might pay to make a separate copy for Lr 5 to play with. At one stage I saw some yucky artifacts in an image that Lr 5 had brought across from Lr 4 but they disappeared when I viewed other images and then came back to the problem image. It was then that I decided to have Lr 5 build its own previews from scratch, just in case.
I dare say that Lr 5 would also mess with .xmp files with or within image files but I set Lr to avoid them and so that did not concern me. I rely instead on the Lr catalog database and back it up often to different media.
For now I am running Lr 4 and Lr 5. I've still got over three weeks before I need to pay to get a licence for Lr 5 and I am hoping that by then any major defects will be well known and I can weigh them up against the advantages before deciding to abandon Lr 4.