Just rented it and finished it. Fantastic game.
The story was surprisingly strong, and there were plenty of genuinely funny laugh-out-loud moments in there, although the game is a bit front-loaded in humor and gets fairly serious (with one or two awesome jokes) towards the end.
The best part, though, was the writing, dialogue, and character design. Every character was amusingly believable and the voice actors (and cameos) were utterly fantastic. The facial animations, lip syncing, and character animation were top-notch (they had a former Pixar employee working on them apparently) and all of the various characters you interact with were well-voiced and designed. I definitely got emotionally invested in these characters.
Also, Tim Curry is just amazing as a villain. Apparently Dio was originally tapped to play that part, which would have been awesome, but Curry just made that character. There are also rumors that he and Osbourne had some friction issues in the studio.
And, the soundtrack; man, is it fantastic. 100-ish handpicked tracks that are unlocked throughout the game, including Iced Earth, Black Sabbath, King Diamond, Dimmu Borgir, Riot, MSG, Saxon, Whitesnake, Megadeth, Budgie, Diamond Head, Slayer, Anthrax, Savatage, Scorpions, Judas Priest, Lita Ford, and a number of bands that I hadn't even heard of that I will now be checking out. Particular tracks are used as the soundtracks for appropriate moments; I got a chill up my spine during a sequence set to the intro to "Mr. Crowley."
The gameplay was fun and reasonably accessible, although the 'real-time strategy' bits are a little tough to micromanage at times and the driving controls are a bit clunky. To be honest, though, what kept me going was the story and the soundtrack rather than the gameplay. The game was also fairly short; I did all of the side missions and a fair bit of exploration/collecting, and was done in about seven hours.
Unlike ODST though, the game didn't feel like it ended suddenly or rushed through anything, and I think the writing is what did it.
Definitely worth a playthrough. Seriously.
GTA IV, both Ninja Gaidens, Fallout 3, KOTOR, etc. are games with outstanding length and substance and I wish more single-player games fit the mold of those. I want to spend 20 hours+ getting engrossed in a game. I finished Batman in a few sittings, barely even 10 hours I think.