My opinion based on the Wrist PDA, MetaWatch, and Pebble
The Wrist PDA still did some things right, IMO, that the Pebble still doesn't. It was a far more powerful device (as far as software - the Pebble probably has more CPU power) that made it a lot easier to avoid a phone pull, and a modern device could do what it did with live sync with the phone databases. However, it was HEAVY, THICK, and had absolutely atrocious battery life (if you made it past a day without recharging, you were doing really well, unless you did the iPod Shuffle battery mod, then you got 2-4 days). Oh, and they broke if you looked at them funny.
The MetaWatch is really a massively flawed device. Battery life is mediocre, and they went for an extremely low power CPU without much local storage at all. This meant that they do almost everything on the phone, rather than on the watch CPU, and the watch acts as a terminal. So, the watch ends up laying on the Bluetooth interface heavily to keep things going, burning through tons of battery. And, because it's literally getting screen updates over Bluetooth, button presses end up quite laggy, especially if you haven't done anything in a while with it.
The Pebble is an interesting balance. There's some UI concerns caused by the lack of a touchscreen (the buttons are good, but I also want touch/stylus for some applications), but the screen is pretty nice, it has enough local processing power to run software directly on it (meaning that the Bluetooth link is used much less, saving both watch and phone battery), and they seem to have a pretty good idea of what they're doing. Right now, with third-party apps running on the watch, and the latest update that allows Pebble<->Android app communication, I'd say that it blows the MetaWatch out of the water as far as capability.
Oh, and both the MetaWatch and Pebble will sync time once they connect. And, the Pebble's low battery warning, I've found on recent firmware to be pretty useful - if I get the warning, that means that when I get home, I need to put it on the charger, but I don't need to worry about it before then. The only time I've run out with recent (1.9+) firmware, I forgot to do that, and went out a second day with it wanting charged, and it still lasted most of the day.