Worth a play but not without its problems.
Let's get the important part out of the way right up front: Brutal Legend is a funny, funny game. I liked the story a lot and was glad to see how well Double Fine have captured the ridiculousness of Metal music and lyrics without making fun of them at any point.
While the fact that Brutal Legend is or at least includes elements of a real time strategy game has been made publicly available knowledge for many months now, the demo released on XBL seems almost to be deliberately misleading, containing the first level of the game, which plays like a linear beat-em-up in the style of God of War, ending with a fairly standard boss fight. That first level is the only section in the entire game that is even remotely like this. The game IMMEDIATELY after the demo ends opens up into an open world where you are usually driving the stunningly named 'Druid Plow'. The game also incorporates very small sections of stealth (for all of about 12 seconds) and the aforementioned RTS style segments which are really the meat of the gameplay.
Jack Black, Ozzy Osbourne, Lemmy and several other celebrities I didn't recognise do an incredible job of voicing the characters. I found myself laughing out loud in most of the cutscenes. This is some of the best comedy that has been managed in a game thus far. Jen Hale (Cortana, Samus, Bastilla etc) makes yet another appearance as one of the female leads and yet again does a fantastic job.
Another high point is the game's use of music. Both the soundtrack selection itself and the appearance of these songs in several key scenes is very impressive and suits the tone of the game perfectly. In a game about music, the music is key and they have absolutely nailed it. One complaint on this front: If your game contains 80 or more songs, DO NOT limit me to 10 or so until I 'unlock' more. They repeat and it's incredibly annoying.
The RTS sections in the game have a very good learning curve and by the end I felt a real sense of accomplishment every time I completed one. I then went into a skirmish battle against the AI on the easiest setting and while I still won, even that was significantly more difficult than any of the battles in the game proper.
Being an open world game there are plenty of collectables and side missions. It is unfortunate that many of the side missions are just the same 3 or 4 somewhat uninspired missions over and over again, introduced with the same 1 or 2 lines of dialogue every single time. Without these missions, however, the game would be offensively short. I did a large number of them during my run through the game and I still finished in just over 6 hours. If you told me that you beat the game in 3 hours on your first play I would probably believe you.
The things Brutal Legend does well it does amazingly well and the music, acting and storyline are good enough that you should very probably play through it at some point, though the game's length and completely bizarre mix of gameplay styles are definitely problematic.