As much as I love the added functionality from jailbreaking (and yes, the enjoyment/usefulness that I get from my jailbroken phone has increased exponentially from that of stock) I have to say that without these extras, the iPhone to me would still be the most full-featured and versatile smartphone/PDA on the market, so I would definitely stick with it.
Besides, as much as I love Windows on my tablet laptop, (sorry, OSX just doesn't do everything that I need to do) I have to say that after owning a BlackJack prior to the iPhone, I completely abhor Windows Mobile and all its crappiness. If you think you've experienced even a SLIGHT amount of lag from running things like Winterboard and tons of other MobileSubstrate based apps on a jailbroken phone, you don't know how good you've got it, because Windows Mobile is just atrociously laggy in and of itself.