Speaking specifically about multiplayer, since single player is a finely polished experience.
I played MP both during beta and after a couple patches of full release. iD has done a decent job of improving gameplay. I still cannot stand being limited to two weapons while playing Clan Arena. They need to have the option for all weapons during that gameplay mode. Death match is enjoyable. It's somewhat fast paced but not as hectic as the single player experience. Physics feel good during small and large skirmishes.
Increase run speed, add full weapon availability to Clan Arena, and allow proper rocket jumping and you have yourself some solid multiplayer. Otherwise I would rate multiplayer 6/10.