For what it is, I got a fair amount of play out of Multiwinia but it certainly isn't anything spectacular. Actually, it's about as simplified as an RTS game can possibly get. There is only one type of unit, and the only real control you have is to give a vague direction to them. Occassionally you'll use an item or pull off a clever pincer attack, but Multiwinia is very simplistic.
Despite being simplistic, or perhaps because of it, a large chunk of the gameplay is a grind. Send your 100 units to kill the enemy's 100 units and try to stop them from spawning more, so you can send in more and take a base that they can no longer defend. Of course, you'll be sending these units in waves, usually immediately after they spawn. As far as gameplay goes, there really isn't a lot there: you'll end up automating most of your units' movements.
There are a few different game modes but only a few of them really work well. King of the hill and generic base capturing are fine. There's a mode that involves capturing bases to fuel a rocket as well, but it's similar to the general base capturing mode. Another mode features capturing statues and returning them to your base (permanently) and while different enough, doesn't really get away from the base/king of the hill modes much. The last type I wouldn't recommend to anyone. It involves capturing a heavily fortified position. One army gets very frequent respawns while the other doesnt, but as there is little to no strategy in this game, it's very painful to play. The attacking team either sits and hordes units, or sends them in as he gets them. Either way, they almost all die for little to no progress against the defender.
There's online vs and it works well enough, but like I've said, the lack of any unique units or strategy really bogs this game down to a grind and you probably won't have a lot of fun with your friends. It's good for a game or two but after that you'll probably want to move on.
On the plus side, the AI is competent enough that playing against it isn't boring, especially if there are several enemy teams. Watching enemy teams duke it out is quite entertaining.
Oh, some really cool/scary things can happen if the aliens or darkwinians get out of control. ♥♥♥♥ those aliens man, they are god damned ridiculous.
Final note? The way items are distributed is pretty biased. I do like the idea that if you want the items then you have to send units to go collect them because you have to decide whether or not the reward is worth the risk of leaving an area unguarded or unharassed (pro tip: it usually is).
I liked it but I think I'm also in a very small minority of people that would. It's just so basic.