FolderGlance works great on El Capitan (and it's still a 5-star app). The thing is, you have to temporarily disable System Integrity Protection (SIP), move a file and run a Terminal command, then re-enable SIP, which takes a few minutes and several restarts, but it's worth it. Here's what to do (these instructions come from the dev):
1. To disable SIP, restart into Recovery Mode by holding Cmd+R at startup. Go to Utilities > Terminal in the menu. In Terminal, type "csrutil disable" (without the quotes) and press Enter. Restart normally.
2. Move the FolderGlance.osax file from your ~/Library/ScriptingAdditions folder to the /System/Library/ScriptingAdditions folder (creating the folder if it doesn't exist). Make sure there are no other copies of FolderGlance.osax on your Mac (check /Library/ScriptingAdditions, too).
3. Open Terminal, enter "sudo chown -R root:wheel /System/Library/ScriptingAdditions/FolderGlance.osax" (without the quotes) and press Enter. Provide your admin password when prompted.
4. Re-enable SIP by restarting into Recovery Mode as in 1. above, but instead type "csrutil enable" (without the quotes) in Terminal, and then restart normally.