I've used an early generation of Extensible Follower Framework for my previous playthrough, in combination with Mounted Followers mod:

http://skyrim.nexusmods.com/downloads/file.php?id=12933
http://skyrim.nexusmods.com/downloads/file.php?id=12175

It worked pretty well. For my next playthrough however I am contemplating using Ultimate Follower Overhaul:

http://skyrim.nexusmods.com/downloads/file.php?id=14037

It appears to replace several mods at once: Extensible Follower Framework, Mounted Followers, Freeroam Housecarls, Followers Can Relax. However, I've got no experience with it whatsoever. Is it any good?

UFO is fine, probably an especially good choice if you like having housecarls as followers since it lets you put your Thane status to use. But just as UFO does some things that EFF doesn't, it also (as of the last time I tried it at least) lacks some features of EFF: the group commands, setting of combat style, gathering, etc. So it depends on if you use those features, if you prefer EFF's messagebox-driven interface versus UFO's dialogue-centric interface, and so forth. I believe newer versions of EFF include sandbox/relax mode for followers and horse riding for followers, so your last line about Mounted Followers/Followers Can Relax likely needn't be part of the consideration.

See also http://www.gamesas.com/topic/1369418-ultimate-follow-overhaul-vs-amazing-follower-tweaks/ for other options/opinions.