The other half-sequel is Rock Band 2. This one is full-priced at $60. If you think of it as just a content pack for Rock Band, it's still not a bad deal, considering there are over 80 songs on the disc and songs are typically about $2 each to download. Although I personally wouldn't have bought every song on this disc if they were á la carte. So what else is there?
Although I didn't initially expect it, they did make all Rock Band DLC compatible. Not only that, but for a modest $5 fee (licensing issues), they made it possible to "rip" (almost) all the Rock Band on-disc songs to your hard drive and make them available to play in Rock Band 2. So, when I put Rock Band 2 in for the first time, I had a library of over 150 songs to play. (The game also comes with a code to download an additional 20 songs later this year, when they become available.)
As far as features. The Solo Tour is gone. Instead, it's just a Band World Tour, which you can play alone (in any of the four instruments, including bass this time) or with others, offline or online. Your band can even have more than just four people in it that shuffle in and out.
Most everything else is the same. You still have almost the exact same character creation controls (I rather wish you could import your Rock Band avatars), although fortunately you no longer have to create a separate avatar per instrument. Basic gameplay hasn't changed; there are still notes sliding down a track, streak multipliers, and white notes that earn overdrive. You still select from a list of playlists at a list of venues in a list of worldwide cities (the interface of which is almost identical to the original). There's still an "endless setlist" to complete.
There is a new "no fail" option, which is great for playing with my 6-year-old son, who can now bang on the drums, miss all the notes he wants, and still play along. I did notice during one song that his note track turned blue, like a guitar solo, so I guess drum solos have been introduced. There's also a "drum trainer" mode, which I believe lets you practice certain beat patterns — I haven't had a chance to play with that yet.
Is it worth getting? If you play online with friends, then definitely. Most of them will be getting it, most likely, and based on my experience last night, it's much easier to jump in and out of an online session. Online band tours are a huge bonus as well. If you don't have Rock Band, it would probably be worth waiting for the Rock Band 2 instrument bundle to be released next month first. Otherwise, the existing instruments work just as well in the new game.
I don't know if it's fair to call these games "half-sequels" or "point releases" or what. What is the threshold for calling a game a true sequel? What, for instance, makes Call of Duty 4 more than just an "expansion" to Call of Duty 2? The new setting? Different weapons? The challenge system in multiplayer? When it comes down to basics, aren't you just running around with a squad of inept teammates who can't seem to advance without their Private leading the way, using the left trigger to aim and the right trigger to shoot? I don't know. It's really more just a "feeling" than a checklist I can go through and say "yep, it has enough differences; it's a sequel".
I "feel" that both these games end up being just "half-sequels". Viva Piñata is priced right for it, though, especially after two years; and Rock Band has enough new content and features to make the price worth it, even after less than a year.