Wow. Open source, free, compression & deduplication, web interface, no client software needed, actively developed... WHY DIDN'T I KNOW ABOUT THIS BEFORE?! I need to try it out!
I have used backupPC for quite a while. It works great but doesn't scale very well when you are getting to the scale of TBs upon TBs of files, 100s of millions of files. I ended up having to run multiple servers.
If you want something along these lines that scales better, rsnapshot works wonders. Rsnapshot is much more simple, as it doesn't create a large pool of files to compare and de-dupe against, it uses rsync's built in deduplication features (so, it doesn't create a "pool" like backuppc.) Techincally, backuppc uses less storage space, so if that's the concern, use it. but rsnapshot is my tool of choice.
EDIT: I will mention that my experience is based on having to deal with very large sets of data across numerous backup targets and at least 4 years ago, I have not tried any of the latest updates that may make this solution better or make performance better. Most people that were in my position probably would have had the funds to leverage an enterprise vendor solution at that scale. For what backupPC does, it was pretty amazing to work with and rock solid in terms of functionality at smaller scales.
No idea why this was downvoted. I don't have recent experience setting up something like this but 10 years ago BackupPC was the answer, and I guess it is still good.
I used backuppc at a place ohh, 8 or 9 years ago now and it was pretty rock solid then (90/10 mac/pc clients) to a linux server with some applescript to manage keys and such.