As I've never has an iPhone or a Blackberry i can't make a direct comparison, but I can tell you my Desire is my second favourite gadget ever (after my PS3).
battery life isn't the greatest sure, but charging it every night has become standard practice.
I don't go app crazy, but even so if I had opted for an iPhone..... well let's just say I'd be much poorer than I am today. The choice of free stuff on Market is fantastic, and if you do have to pay it's not usually much more than 2 or 3 quid.