Seeing as I have a weekly meetup with 20+ to play games, split screen in this situation is undoubtedly the shit. Well, our meetings wouldn't exist without split screen, so there's a two-way relationship going on.
I'd say that split screen is more sociable than online play because when you're online, most people are anonymously waving their dicks about, safe in the knowledge that nobody will be able to put a face to that name in a sea of millions. And face it, you get more dickwavers than nice people online. When everyone's in the same room, nobody's going to do that because everyone is friends already. "That guy" isn't laughing at yor face behind a mask, he's complimenting you on your sterling efforts, even if they're more futile than paper tanks.
I won't say splitscreen is entirely faultless, though. The machine now has to work up to 4 times harder to draw stuff for 4 cameras (but that's not a problem as nothing pushes the limits of today's machines yet!) Your screen is now squashed into the corner, making seeing stuff markedly difficult unless you pull your chair up and ruin your eyesight for years to come. Plus the business of sneaking around is buggered as everyone can just peak at others' screens.
You can LAN it all together, but that requires everyone to have their own machine and copy of the game (I just lost The Game), then you need lots of cables, several televisions, a room large enough for all that, and then maybe get an air conditioning unit to stop everyone dying of heatstroke. When you've ticked all those boxes, I'll give you two thumbs up.
The most realistic thing I've come to witness is a mix of the two: two medium sized TVs, two consoles, 8 pads and a jar of cookies. Works excelently with 4 on 4 games.
If there's the potential for split screen, just do it; it'll add just as much value than online play, and isn't hard to do, anyway.