*CHOO CHOO* OI! DA RAPE-TRAIN IS A'COMIN'!
Now that we've gotten that out of the way...
The Battlewagon is the Ork equivalent to a Space Marine Land Raider, in terms of both size and sometimes durability... you never know with Orks. Basically the most badass and ramshackle pimpmobile ever. Seriously, you'll never see another vehicle come in more lovely shapes and sizes than a Battlewagon. It's typical role is to ship mobs of Boys to the front and carry ridiculous Orky gunz, most notably the Lifta-Dropa; a goddamn gravity cannon that flings tanks like toy trukks. Owned most commonly by a Mekboy wanting to have something to carry his loot, or a Warboss and his retinue of Nobs. While each Battlewagon can be unique, they most commonly have a giant bulldozer blade and big gun turrets on top. Wouldn't be surprised however if they had giant stereo systems blaring Iron Maiden
It represents what all Orks are supposed to be: loud, bombastic, and ded killy.
Battlewagons are, sadly, meh on the tabletop
Its main issue is its price tag of 110 points. Remember, that is just for the bare chassis, no weapons or upgrades to it. If you're not careful with it, it can rack up points stupidly fast. Also, it can't be a dedicated transport for your Boyz, so you'll have to eat up a Heavy Support slot in order to drive them around. Sure, it has 14 FAV, but if your opponent drops his combi-melta Termies in behind your juicy Back Armor 10, bad things will happen.
On the bright side, you can stick ridiculous amounts of ordinance on this thing. For 5pts each, you can put four rokkits or big shootas on it. You can slap on a kannon, zzap gun, or lobba for some anti-infantry shots. Then we have the hilariously named killkannon; S7 AP3 Large Blast. A Battlewagon can spew out some of the most powerful dakka Orks have. For extra fun, throw a mob of 20 shoota Boyz in there and do an Orky drive-by. Waaagh Tang Klan ain't nuffin' ta fuk wit.
Oh yeah, its transport capacity is fucking twenty Boyz. Shit, your opponent will do everything in his power to shoot it off the table before it gets to his gunline, especially if it's full of MANz and a Warboss. This is why a Battlewagon can be a good DISTRACTION CARNIFEX. Though not as viable as it used to be with that price tag, it can be a pretty good suicide missile with a killkannon or a bunch of big shootas.
On a side note, it has one of the coolest vehicle models in the fucking game, and still has the best scratch build potential out there. Go out there, and make your own little scrappy fortress on wheels!