I've not yet played multiplayer. It is simply not convenient for me to do at work. I will often need to pause during the night, and there has yet to be invented a multiplayer pause function that works in a fair and unobtrusive way. Luckily, it pauses in single-player, which is a consideration that not all games grant (I'm looking at you, Dark Souls).
What I've been doing so far is unlocking characters and trying them out on easy bot battles. I've managed to get all of the characters from the base game (though there are still five more I could get if I purchased the expansion . . . which I'm tempted to do because this is a pretty fun game and having blank spaces in my menus bugs me) and I've played through at least one match with all of them.
They're a pretty well-designed bunch of characters. No two felt exactly alike to play. I'm not actually good enough at the game to appreciate the deep differences between them, and the easy-bots are indeed easy enough that clever combos and min-maxed builds were never really necessary, but there were a couple I liked - Derpl Zork, the cute little alien in the roly-poly power armor that can enter siege mode and lay down an astonishing amount of firepower at the expense of mobility (and for some reason shoots cats out of his gun) and Voltar the Omniscient, a weird psychic squid creature who can heal your allies and destroy enemies with floating defense drones that can double as grenades.
I think I could easily play bot-battle mode indefinitely. The levels aren't really complex enough to make it a great platformer, but it's fun to go toe-to-toe with the AI in an all-out brawl, and since repetitiveness doesn't really bother me as such, simply grinding profile levels in pursuit of all the unlockables is more than enough to keep me occupied.
That being said, I will indeed try multiplayer mode at my earliest convenience. I'm sure that some of this is simply due to the fact that I've been playing on easy, but I saw the AI make some pretty silly choices (such as being too willing to retreat when I'm at the inner base, and thus no use at all in final, end-of-match defense), and I imagine that if I'm to see any decent tactics at all, there will need to be a human intelligence behind it.
I think I'll play my first multiplayer match today, before bed, then get some more practice tonight at work (bumping up the bot difficulty to level 2, at least), and then dig in to multiplayer in earnest tomorrow, on my day off. Hopefully the more experienced players don't eat me alive.