I'm not sure I'm finished tweaking yet, maybe I'll upload it to the sharing servers once I'm done. Although given it's not a replica anything it probably won't have many people who'd want it (and it's using stock items in fairly straightforward ways, too).
The racing karts are also alterable, and again there're many replicas people have made - Dukes of Hazard car, A-team van. Mine's purple. Bet you're surprised?
You can make tracks, too, but I haven't got round to trying that out yet. J has played a bit with the track editor and says it's very simple - at the most basic level you can just drive a track-layer around to make your circuit, tell the editor to autopopulate scenery and hazards. And there you go, you have a track! Like with the mod & kart editors you can also do everything yourself, and tweak the stock elements and generate something different & cool.
And that's all just an aside to the real game - just an elaboration of how everyone likes to customise their ingame avatar. And the custom tracks will hopefully give the game a lot of longevity, it's so easy to make something playable so hopefully lots of people will make them.
But the real game is, unsurprisingly, a racing game. There's a career mode which I've done about half a dozen races from (and that's all I've played really) and I think single-player quick races, and various multiplayer racing options. I've not tried any of those, but I've watched J playing - there's XP races (more "serious" where you get points for winning), casual races (no points, more jump in & play), split-screen racing for local and online multiplayer (I did play one of those with J in the demo), hot lap races (time trial, each day there's a track & you are competing against the whole world). The career mode ramps up the difficulty quite quickly - I'm still doing OK, but I suspect I'll hit a brick wall before the bit where J has got to. If nothing else because it's another of those games where I get so intent on what I'm doing that my hand locks round the controller in a death grip & that does my fingers in after a while. So I can't just play for hours, I need to limit my playing sessions.
The AI is pretty good, with a tendency to dogpile you if you're doing well, and leave you in the dust if you're not driving well enough. Having said "dogpile" I realise I forgot to mention the weapons - a bit like wipeout you pick them up from the track. And there are speed up pads like wipeout too. I think there's 2 sorts of races - "action" with weapons and hazards (like the Devastator that can drop & squash you), and "pure" which only has the passive hazards like jumps/cliff-edges and no weapons.
The only flaws I've found so far are the long loading times - it apparently generates the mods/karts on the fly rather than being pre-rendered so that takes time (this is to reduce download times for multiplayer games - what you have to download for the other player's mods/karts is an instruction set for how to generate it, rather than the thing itself). And the XP races seemed hard to get underway when J was trying - you need 4 players minimum and you only really have a pool of 24 (random) players in your instance of the ModSpot, and there's no advertising of a waiting game (you can do it yourself by saying something on global chat, before you go in, but the game itself should maybe do it) so you can sit there for a while before enough people join.
The game is kind of setting itself up as the Little Big Planet of racing games - sense of humour, single-player but with the real meat is in multiplayer, extremely customisable characters & a lot of user-generated levels for longevity. I think it may even end up with more user-generated content because you need less of an "idea" for it - LBP is very open, you can do anything you can imagine, so you need to have a goal before you start. This is creating race-tracks, so that gives you some structure rather than a total blank canvas which is easier to just play with and maybe make something great (or maybe it's just me that finds the total freedom so intimidating?).
Definitely recommended :) But be prepared for the long loading times for tracks.