We upgraded our entire institution from Endnote7 to Endnote X (network version and individual licenses on laptops) about two years ago. We found EndnoteX to be very stable with the only annoying bugs being incompatibility with some plugins including Flashpaper and Snag-it and the fact that formated documents on Mac and Windows were not compatible across the two platforms (these two issues were fixed for most part X1) .
The formated documents and librarys are not backward compatible but we had no issues that I am aware of with the library conversion.
We found that it helps to unformat documents in Endnote 7 prior to migration, or to unformat them in EndnoteX and reformat them. Once formated with Endnote 8 or above (don’t get 8, as it was a very buggy product), you cannot handle them in Endnote 7 anymore. If you collaborate extensively, even with the travelling library and both authors using post Endnote8 products, it is probably still safest to share unformated documents. When ready to submit, share the libraries or have one format a copy (ignoring items not in their library) and send to the other for the final Endnote formating with their library.
Perhaps someone can comment on similar experiences going from 7 to X1 (or now X2). On the whole, I believe most people at our institute like X better than Endnote7. In a recent quick test, I didn’t find any issues openning, unformating and reformating Endnote 7 documents in the newest X2 version. I also had no trouble converting a library of over 5000 records.