I mean if you own Diamond/Pearl are the updates on Platinum worth the purchase or not?

It's entirely up to you. I say its well worth it for the cool additions and the Frontier. Not to mention Origin Giratina and Sky Shaymin.

Yes it well worth it if you Collect or enjoy pokemon games,Bought it when it came out

Yeah I do collect them,I have almost all of them.

I love the new features
the wifi plaza's fun lol

It depends on you. There's a lot more to do outside of the main story and over wi-fi. Also the annoyingly slow surfing speed in D/P has been increased. Additionally, if you like to battle with others a lot you'll probably welcome the increase in the battling speed.

I got it regardless, being able to catch articuno moltres and zapdos is already a plus, although you could transfer them from fire red/leaf green, but that's what i don't have.

Still playing Tales of Hearts so won't be opening the box sooner though I can't wait to play Platinum

If you love Pokemon then yes.

It was well worth it for me, but then again its a case by case thing - I have purchased atleast 1 of each set of pokemon and then the 3rd out of every pokemon gen so it was a no brainer for me.

LOL, me too.