A drawback of all these packages is that each file (download) item can only be referenced by a single category. It's true that you can create loads of categories and sub-categories, but each file can only belong to one of them at a time.
Fortunately, jDownloads allows you to use several other fields - and even create custom fields. With a little research and effort, I found that I was able to expand the search routines to cover as many fields as I wanted. This makes it - for me - by far the best of the other packages on offer.
The English language is slightly curious at points but, coming from a non-English developer, it's understandable and perfectly forgiveable.
It also relies fairly heavily on tables for display purposes, but there is easy access to the relevant templates and nothing to stop you substituting some decent CSS formats instead.
Overall, if you're looking for a really thorough and flexible component, I would strongly recommend that you start with this!
I have used some of the competitors (Remository, PhocaDownload, and jDownloads) but none of them really met the demands of a new site that I am working on. After reading through the description and favourable reviews of Skyline, I thought that it might be worth trying for $25.
When I first installed it and ran it, the software reported an error. It was nothing serious - a deprecated command in one of the licence functions. I was able to find and fix this myself, but it wasn't a great start.
Overall, this component doesn't really offer anything more than any of its free competitors and certainly didn't meet the needs of the site that I'm trying to produce. I would caution anyone thinking of paying good money for this to have a serious look at one of the other packages first.
It seems that the service provided by Skyline is pretty good. I haven't bothered with it because I was able to fix the problem myself, but if you're not familiar with PHP, it may be worth paying something to get help when something goes wrong.
However, I've always found the non-commercial suppliers to be very helpful when there are problems so, for most people, I'm afraid I really wouldn't recommend paying for this package.
The package as it came wasn't quite what I wanted but, after hunting through the code, I was able to customise it to more or less what I'm after. The code itself has some useful comments in it but finding the files you need to amend is not so easy if you're not an experienced user. I also found that no one seems to have cracked the complexities of the user permissions. There are several queries posted on the Forum but not so many answers.
But the bottom line is that this is a useful component if you want a simple, straightforward download facility. If you want more than that, then be prepared to pay for it!