I have a BlackBerry Curve on T-Mobile, and generally think it's the best mobile email client I've ever used. There are a decent number of downloadable apps, and OTA J2ME provisioning is actually pretty awesome. In terms of "hackability", it's tough to find anything better than a full-featured, reasonably-fast Java stack, since you can do development and deployment from your choice of platforms and hosting providers.

However, the Curve firmware (at least at OS version 4.3) has two major flaws. First, it's intermittently crash-prone and slow when using 3rd-party applications. Second, it locks up after a few minutes and has to be hard-rebooted when I try to use it as a Bluetooth modem for my Mac, which is especially annoying since a full restart takes more than five minutes.

Regardless, I still think I made the right choice in selecting the Curve over an iPhone, for one simple reason: UMA. Being able to pay $20/mo. for unlimited calling over any WiFi network has already paid for itself many times over when I've been outside the US (or even in rural parts of the US) and avoided having to pay roaming fees.