...it's SO going to be worth your purchase(s)!! I got to play with my friend's game at community college and I had a blast with it. The battle sequences are a lot faster. You can hear wind blowing, water sloshing around (if you are near a body for water such as a river, lake, or ocean, even inside caves there is the sound of dripping water!, your footsteps on normal ground, the sound of grass rustling as you walk through it. Since any Pokemon follows you, the first Pokemon in your party is the one that will follow you outside its Poke Ball. There is A LOT more detail given with the outside world; Pokemon Diamond/Pearl/Platinum were just the engines in which HeartGold/SoulSilver were based upon, but more fleshed out; there's lily pads in small ponds or lakes, flowers appear in more places; the leaves in and around Ecruteak (the autumn leaves) are scattered upon the ground...
The music is a lot better as well. It seems more instrumental than what was found in Diamond/Pearl/Platinum; the battle theme has more of a jingle to it. The Pokemon animations seem a lot better than just simple stretching and bouncing around.
All in all, you fellow gamers are going to LOVE it once it hits the states. I know I will. March 21st 2010 can't come any sooner! That's the tentative release date, seeing as Nintendo said it launches in the spring of 2010 and Platinum was released (I THINK) around that date this year too.
