Pokémon Platinum is probably best summed up as the game that Diamond and Pearl "should've been."
If we look at Platinum in a vaccuum, as though D and P didn't exist, I'd give this game a 9.0 or more. For a DS game, it excels in utilizing the console to its pinnacle; the game sounds are strong, the graphics and visualizations are superior and it presents hundreds of hours of gameplay for a significantly lower price than its other-console counterparts. So, for those new to the franchise, they will probably not be disappointed; though playing Pokémon for the first time is probably unlikely for the majority of players. If, though, you are in fact new to the franchise, you won't be disappointed in terms of value and performance; and if it's for your child, you will find that the game offers enough guidance and tutorials that can help young children make their way through the game.
This point is where the game truly shines: most games have a target demographic, the Pokémon has the unique ability to traverse many.
However...
Having predecessors in the series in Diamond and Pearl, Platinum has to struggle with the problematic quandary of "do we completely revamp, or merely make additions?" In this case, the Poké-creators have decided to tweak instead of re-invent the wheel, so to speak. Some changes are excellent and were much needed: the ability to purchase poffins and berries instead of creating and growing, the inclusion of a wider range of Pokémon to choose from, graphical enhancements at previously-weaker places and a higher difficulty scale in places that needed such.
Ultimately the game's storyline, world and strategy are largely unchanged. Very little new dialogue and only a few token NPC's have arrived, and if you HAVE played D and P, you will probably feel a bit disappointed in just how little has changed in these areas. In summation, Platinum can probably be best-described as what Diamond and Pearl should have been to begin with, but it's still much fun. You gotta get 'em all! :)