Surprisingly enough, I can easily identify my worst mobile experience and unfortunately, it is same for all mobile devices that I ever worked with.
On all devices from Sony Ericsson, Nokia, HP and Raon (and actually all other laptops I used, mostly Lenovo Thinkpads) Bluetooth stack just crashes after some period of use. It takes longer on some devices, or considerably less on others, but anyway, Bluetooth inevitably crashes leading to the need to restart, reboot, sometimes even take out a battery from a frozen device.
Pocket PCs before WM5 were notorios for lack of system drivers memory, and after half a dozen of BT On/Off cycles, BT drivers would fail to load to memory.
Sony Ericsson phones would hang if phone call came during BT connection (file transmission, dialup, syncing)
Nokia handles well under load, but after several cycles of using it with Everun as dialup modem, there comes a moment when it stops connecting to UMPC, until rebooted.
Raon, just as any other XP or Vista computer, doesn't handle well going through several cycles of sleep or hibernation and guess what, at some moment when you need to get online, BT either doesn't start, or connects but doesn't load anything from the phone.
It is my biggest disappointment, that despite being a very old technology, BT stack implementation still remains buggy across various manufacturers. One day I will switch to built in HSDPA or WiMax just because of this.