Advatages

• semi-cheap: TI-8x's can be had from half.com for $25-50 (eg. compare this to the cost of putting together a detachable LCD + keypad that supports VT100, then add in the pain of soldering something like this together, and it really can't be beat). Not to mention my family has a number of these laying around...
• very easy: hardware interface consists of two resistors and a 2.5mm jack. Clock is determined by both sender and receiver, so it'll never be too fast. It only takes up two pins on the PIC, and it doesn't use the UART so I2C/serial is still available for use. The TI-8x is detachable and just leaves a small plug, so black boxes with PICs in them that don't normally need a display can be pretty small. The codespace on the PIC is pretty small.
• quite flexible: supports VT100 (with addon programs on the TI-8x), allows most ASCII characters to be typed
• extra memory: The TI-86's 128 kilobytes of flash memory is gargantuan compared to the 16F628's 352 bytes of memory (624 bytes on the 16F877!! impressive, huh?), and its memory is pretty easy to interface to (though I'm not quite sure how to do keyboard/screen IO at the same time as memory use)

Downsides

• It's only battery powered (unless you hack it), so debugging PIC code with it may burn through batteries.
• It's not a good solution for projects that need a dedicated LCD or keypad (though I'm trying to stay away from expensive electronic projects in general, so this may not be a problem)