Gran Turismo is simultaneously one of the most comprehensive portable video games ever made, and yet something of a let-down. Coming from a distinguished line of driving simulators long-revered for its pin-point driving physics, supreme track design and immense depth, the series' first portable foray has all of the above in spades, though its open-ended nature and loose structure are both a blessing and a curse.

Its Career mode is all very different from what's gone before. Whilst previously you'd have to slave away earning licences before even thinking about competing, buy a cheap car, scrounge away for a while until you could afford a better one and then move on from there, Gran Turismo on the PSP does away with structured tournaments, in favour of single races. With each circuit you're tasked with working up from Rank D to Rank S, and you have the freedom to race any vehicle in your possession for however many laps you should so choose.

It's understandable that series aficionados may be put out by the lack of focus. After all, the heavy mix of tournaments, endurance races and model-specific cups are the meat and potatoes of the average Gran Turismo experience. However, what's often overlooked in such circumstances is that the game offers accessibility without sacrificing the depth of its racing - it is, after all, GT-on-the-go, and being able to fly through a couple of three laps races is more ideal on a train journey than perhaps a seven race series. There are genuine positives too - the eternally tiresome ritual of Licence Tests has been replaced by a similar though crucially non-mandatory set of "Driving Challenges" which are a nice means of getting used to the game with the added bonus of monetary rewards for good showings.

What should be recognised off the bat though is how great a technical achievement the game is. Bearing in mind the PSP's relatively modest specs in comparison with its home-console contemporaries, it's awesomely ambitious. Developer Polyphony Digital pulled no punches, basing the physics engine on the PS3 GT5 Prologue, resulting in a game that, but for a few periphery sacrifices, runs beautifully.

The frame-rate is silky smooth, the action blisteringly quick, the handling challenging but fair. The cars are typically outstanding; both in terms of how they look and in the way they pitch under braking and warble when traversing curbs. Even within its deficiencies, there are signs that elements have been cleverly prioritised. The road surfaces look fine and close scenery looks grand; prominent backdrops such as those seen in the Grand Canyon course have been beefed up and look great, whilst some of the trees, which appear flat on the replays, don't give such an unflattering impression when whistling past them at speed. Most of the uglier-looking buildings and advertisement hoardings have been kept out of the player's primary visual focus - although tracks with tonnes of the above - most notably the Monte Carlo street circuit, find it more difficult to sweep mediocre visual elements under the rug.

The odd bit of low-fi visual design is likely due to data compression, as what Polyphony have managed to cram onto one small UMD is almost beyond belief. That there's an incredible 800-plus car roster is almost taken as a given these days, but it's still worth noting that this includes the likes of Lamborghini and Ferrari, after their Need For Speed exclusivity rights presumably lapsed, and elsewhere everything from Fiat Pandas and Volvo Estates to Bugatti Veyrons and Dodge Vipers. But it's the tracks which are really special; there's a staggering 45 individual circuits in total - ten more than even the game's box gives it credit for - some of which are admittedly alternative or shortened versions of the main tracks (Suzuka and Fuji for instance), but the number still doesn't take into account mirrored courses. The vast majority of the racetracks on show featured in GT4 but there's also the eminently appreciated appearance of the Valencia MotoGP course from Polyphony Digital's motorcycle sim Tourist Trophy.

Course design is, as ever with GT, almost flawless. Real-life racing royalty such as Suzuka or Laguna Seca allow you to appreciate what makes them such unique challenges, whilst GT's own creations, such as the colossal Grand Valley Speedway, Autumn Ring and Trial Mountain, all seem to get better with age, presumably as the technology allows all of their little nuances to feel that little bit more "real". Not only is the player spoiled by the array of race tracks, there's all manner of dirt, snow and ice rally stages to powerslide around. Markedly different but in many ways just as engaging, you'll be scrabbling round corners desperately searching for a purchase, allowing you to put the power down and catapult away. Virtually every WRC-winning car of the last twenty years is present, including many of the infamous Group B/S supercars of the eighties like the Lancia Delta, Ford RS200 and Renault 5, which are bags of fun to drive. Suffice to say, there's something to test every facet of your driving ability here.

A couple of long-standing criticisms linger on however. There's still no crash damage, and whilst it's still questionable whether the inclusion of such a feature in an already-challenging game would necessarily enhance the playing experience, it remains the one glaring omission from a game that aims to ape real-world physics so minutely. A broader trouble is that GT more than ever paints itself as a driving simulator rather than a racing game. Fans will know the real satisfaction of the game lies in mastering both car and course, but the fact the competitive element is so negligible doesn't help matters. The number of competitors in a race has been trimmed from six to four, and though the A.I. is marginally more conducive to close racing than before (they're given cars that roughly stack up against your own choice), they still chug around like automatons. Loading periods are just a little too frequent for comfort as well.

Elsewhere, GT's recent struggles to associate itself with a soundtrack of any distinction continues with another incidental selection of rock songs buzzing along harmlessly in the background. Nothing terribly unexpected there perhaps, but more disappointing is the lacklustre quality of the sound effects - engine notes in particular sound whiny and not up to the series typical high standards.

A lot hinges on whether you enjoy the mechanics of driving in GT, because if you do, it's the kind of game that a player could quite easily dip into for months on end. It's hard to see gamers playing it solidly for any great duration of time however as, aside from saving up enough credits for a dream car and attempts to reach Rank S on all the circuits, there isn't really a concerted, motivating factor to play long term. It is however one of the PSP's best exponents of WiFi play, and with adjustable handicaps, they ensure superior, close racing between you and a friend - provided you race each other in the same car, that is.

If you're looking for bumps 'n' scrapes, wheel-to-wheel racing then in truth, this is unlikely to leave you fully satisfied, though when it comes to a wealth of cars and superbly-designed tracks; it's a veritable trove of driving delights. It's mostly stunning to behold, though with the odd graphical concession, you can't help but wonder whether the mountainous amount of content was a case of Polyphony Digital biting off more than the PSP could chew. Nevertheless, with tuning options and structured progression traded for accessibility, it still feels every inch a Gran Turismo, even if a couple of troubles stop it from being as defining as its forebears.