Well I've played through the Japanese version of the game fully. It has many improvements and additions over the earlier Diamond and Pearl versions of the game, although most of them are minor things, so anyone who has played the older game to death needn't feel that Platinum is an essential purchase. If you can't get enough of Pokemon though, this is the best version so far, of course, and anyone who did not play the previous game could not choose a better value game for their DS than this! It is packed full.
The more substantial new additions in Platinum include an extended storyline, new transformations for a few key pokemon (see Giratina on the box cover), new animations for pokemon and trainers in battle, three of the gyms have entirely new interior design and puzzles, enhanced visuals or new design for many locations, more pokemon to catch (the Sinnoh region pokedex has expanded), the option to rematch key characters including gym leaders at a cafe, the massive new Battle Frontier area that provides hours of difficult challenges for advanced players, and the poketch on the lower screen now has two buttons so you can cycle through apps both ways!
Oh, and for those who care, both Dialga and Palkia can be obtained in Platinum, whereas in Diamond and Pearl you could only get one or the other depending on the version. The opportunity to catch both of them comes later in the game, because your first visit to the Spear Pillar atop Mt Coronet now leads to an encounter with Giratina in another world! Also the legendary birds Articuno, Zapdos and Moltres can each be caught after the player has defeated the Elite Four and visited Professor Oak when he goes to Eterna City.
Aside from content changes, gameplay is identical to previous pokemon games, so you know if you like it already. If you've never played a pokemon game, it's worth checking out and this is as good a place to start as any. x