i sold my iphone 4 mainly cos i was bored of it (having owned previous iphones) and it was the worst mistake i made!!
i currently own a HTC desire HD and its shockingly bad, terrible battery life, crap interface and i dont like it.
I cant afford to buy another iphone 4 outright and my contracts not up til; december, so im looking at getting a blackberry.
All i say is do it if your getting a good offer.
one thing to point out though, you mention you've put aussie sim cards etc inside it, beware that the iphone 4 doesnt take a regular full size sim card - it uses a Micro Sim Variety which is smaller than a normal full size sim, see pic