Which EndNote version to use?

You should continue to use EndNote 7 if you are sharing EndNote libraries or computers with colleagues who use EndNote 7. EndNote 9 and EndNote X are not backwards compatible with EndNote 7. An EndNote 7 library can be converted to EndNote 9 or EndNote X but an EndNote 9 or EndNote X library cannot be used in Endnote 7.
EndNote 8 had a large number of bugs and should not be installed.
EndNote 9 and EndNote X have Unicode support for non-Western languages.
EndNote 9 and EndNote X allow an unlimited number of references in a library. EndNote 7 allows 32,000 or 32MB.
The same import filters and connection files can be used in EndNote 7, EndNote 9 and EndNote X.
EndNote 9 and EndNote X can use EndNote 7's output styles. EndNote 7 cannot use EndNote 9 or EndNote X's output styles.
Endnote 7, EndNote 9 and EndNote X all allow links to PDFs with slight differences in the method. X allows easier sharing of an EndNote library's linked PDFs between different computers.
EndNote X can save an EndNote library and associated PDFs as a single compressed file.
There are some differences in references types between versions.
EndNote X allows empty fields in a reference to be hidden and up to 8 fields (instead of 5) to be displayed in the reference list.