I've been using LR Mobile since the week it came out and have a few experiences to share--maybe I'm not understanding perfectly..
Last week, I updated my iPad to iOS 8.1.3. When I tasked it to enable offline editing for several collections, the program crashed. Restarting it caused it to crash again. Restarting the iPad fixed it. Uninstalled and reinstalled LR Mobile and seems to be working ok, however I need to select enable offline editing for each collection. The way the program reacted I suspect something in the software cannot handle a large number of requests to enable offline editing simultaneously. I believe, in this case, I had asked it to enable offline editing for about 15 collections and about 7000 photos. It seems ok 1 collection at a time.
Occasionally, maybe monthly, I will go to LR Mobile and find I am no longer signed in (occurs on both iPad and iPhone). It could be there was an update from Adobe (current version is 1.3.0)
The offline editing button on the front of the collection has inconsistent behavior. If offline editing is off, selecting it raises a window indicating how much storage is required and available and you have to say go ahead. If offline editing is on, selecting the button turns if off but never asks if you want to delete the photos that were already downloaded or not. I also find the two different states of the button are not particularly noticeable--I just sat down today to figure it out and can see the arrow drops when offline editing is on, but maybe adding a white background or something would make it more obvious what happened.
I don't think there is a way to choose a subset of collections for different devices. So, if I set up the collection on the Mac, both devices get it, although I can select offline editing independently. Selecting "Remove" on an iOS device simply removes the synch with LR marker from the desktop (and causes the collection to no longer be available on any iOS device).
Not too sure what the value is if you don't enable offline editing. Download is too slow to be particularly useful. I could see a photographer might want to download only the client set s/he is working with, but for me, I need to be able to look at all the photos while not connected to the web.
I think a pretty consistent thread has been to synch smart collections, and I sure vote for that!