Collections are so poorly implemented at the moment so I barely use them.
I mean, they could be the most wonderful feature of the whole game almost, but I'm still waiting for it to happen. I'm sure they're working on it... right?
I was amazed when renaming them was recently added, I was like 100% sure they were gonna rebuild the whole collection system instead of trying to band-aid the thing they have now.
And just so this doesn't sound ONLY like a angry rant, I'll put a list of things I'd like to see here:

Shareable collections that you subscribe to, always up to date, supports multiple authors.

1. Do you even realize how heavenly this would be? If anyone asks "give me your favorite DT maps" or anyone has a list of "dnb music?", you just give them a link to a collection file. And they run it, and it just works? oh man, it's so convenient.
2. Beatmap entries in a shared collection that you don't own will still show up, but will be gray and have the "click here to download text" that you're familiar with from multiplayer.
3. Ability to hide any entry in shared collections that you aren't the author of.

A way to add, move and remove more than one beatmap at a time from collections. Managing anything right now is impossible.

1. Be able to see in the song selection screen how many collections a song is in. (optional toggle for this to be visible).
2. A collections listing on the webpage, where the top rated and top subscribed collections are listed.
3. Auto collections that work on a filter basis, the same way you filter the song selection right now, you could create a collection that will automatically include anything that matches its preset keywords, ex. "bpm>=195 bpm<230 length<=180 AR>=9 star>=4.55".
   
   1. You should be able to set a filter to any collection though, not just the auto ones. So if you for example have found a good huge shared DnB/dubstep collection but you only want it to show star>4 at all times, that should be possible.

Make collections of collections, call them Libraries or something. That handles duplicate entries sensibly. With shared collections around, you might want to smash some of them together.

1. Make it possible to construct these libraries while browsing the collections listing webpage and the download the library as a .osl or whatever file.

Bring back the "Selection options" menu button to handle all the visibility toggles you could want for these things to work well. Hide maps you don't own, show hidden maps etc.That would be a start. Just a quick brainstorm, I'm SURE the devs have thought of this, I mean I don't actually know if they have, but I expect them to have looked at the collection system and thought "yeah this is shite, we should do something"
Like has no one ever contructed a playlist in one of the hundreds of music playing programs/apps/websites? Most of them do at least TRY TO MAKE IT EASY TO SHARE SHIT WITH YOUR FRIENDS.