Mechanically, then, the game is a perfect piece of rhythm-action design. The difficulty levels are distinct and appropriately pitched, and there's good consistency within each - there's not much of a leap between particular songs within the same difficulty level. For advanced players this is one of Unplugged's chief weaknesses; there's no Buckethead here (the closest is Freezepop), no giant leap between the songs at the beginning and the impossible challenges to be faced at the end of the setlist. As in the home console games, Rock Band's ultimate challenge (the Endless Setlist) is more a matter of endurance than pure skill.
The 41-track selection of master recordings is impressive, and there's a DLC store to expand it well into the future if you're willing to pay. The tracks in Rock Band Unplugged aren't necessarily here because they're good pieces of level design, but rather because they're popular tracks (most of them are RB1 and 2 favourites), and though that makes them fun to jam along to, it means that this isn't a game as precision-engineered as Amplitude. Still, the music is unquestionably better, so it's a more than acceptable trade-off.
Annoyingly, there's a lot of repetition in the tracklist thanks to the World Tour single-player system, which is identical to the previous games'. You'll have heard Livin' On A Prayer, Aqualung and Drain You so many times by the end of the first three hours of your band's career, you'll want to kill someone, like Bon Jovi, Jethro Tull or Kurt Cobain (oh, wait). The tracklist itself may be good, and varied, but because only a small selection of them are available at the beginning and Rock Band has always been hellbent on artificially lengthening the setlist by making you play the same songs in different gigs across the world, you'll be hearing them far too often.
This isn't so much of a problem when you're only playing the game for 15 minutes at a time, of course, rather than driving through the entire tour in three days for a review. The real problem with Rock Band's tour system is the way that it encourages you to stay within your comfort zone, punishing failure by robbing you of fans and stunting your progress rather than letting you play at your upper limits.
Outside of Tour Mode, Rock Band Unplugged offers very little. There's no multiplayer, which seems very odd indeed, and no online scoreboards either, so your rock achievements are for you and you alone. There's an unlock-all cheat built in for Quickplay, which is thoughtful, and a warm-up mode that lets you play in a no-fail setting and without phrases (if, for example, you want to go through a whole song playing just the drum track).
There's also Band Survival Mode, which removes phrases from the equation entirely and has you switching between tracks at random trying to keep them all in the green. It's more of a fun extra than a real alternative mode. The lack of multiplayer and other modes is dispiriting, but for most of us a 41-track selection in Quickplay and fully fleshed-out World Tour mode is probably enough to fit our needs for a portable music game. When the main gameplay mode itself is so thoughtfully constructed, the absence of extraneous modes doesn't smart for long.
Rock Band Unplugged is a lovely-looking, unique and tightly designed rhythm-action game built of equal parts FreQuency and Rock Band. The track selection's good, it's far from a lazy port, and its similarity to Harmonix games past makes playing it an unexpectedly nostalgic pleasure. Multiplayer might have made this an essential PSP purchase, but for 25 quid or so it's worth the money regardless.