The reason to get this game is the incredible single-player campaign, but honestly, you might be one of the people who really dig the multiplayer. It's been a mixed bag for some because it's not pushing the envelope on anything and it's a bit limited next to the campaign, but I think it's fun. It reminds me a lot of UT '99, albeit with limited loadouts (but you can switch loadouts during respawn, which is instant; you can also activate different perks during respawn).
The action is very fast-paced, jumping and double-jumping everywhere, shooting rockets and other high-impact weaponry, picking up health and armor packs. The rocket launcher is actually the default weapon of the default loadout, lol. When in doubt, throw a frag grenade for those sweet post-mortem Redemption kills.
I'd recommend watching some MP footage first (ideally from the final game), but it's a lot of fun. The multiplayer is an extra in this game, but it's one with many well-made maps and modes, some cool mechanics like the rune that turns you into a rampaging demon of your choice, and each match ends with you being showered in rewards (i.e. new perks, weapons, color schemes, cosmetic pieces, taunts, etc). Also keep an eye out for the chainsaw!