Being the runt that I am, I was a bit worried that this fat watch would look a little bulky on my puny wrist, but thankfully it's not as bad as the side-shots would have you believe; it looks and feels like an ordinary watch.
Unlike other pieces of technological wearware this piece of kit actually looks reasonably good, it's not as stylish as the executive edition, but it scrapes acceptable business attire while retaining geek cred. Incoming call, SMS and email alerts are sent directly to the device via vibrations and a scrolling message. I can control my media player and volume from it, and most importantly it panics if my phone goes out of range.
The Bluetooth does sap a fair amount of power, but when the battery drops to critical levels the link is automatically severed, allowing the thing to continue for several more days without a recharge. I've gone for almost two weeks without recharging it which is far longer than I expected. It is only usually removed from my wrist twice a week, for an hour while recharging and for two and a half minutes during sex (apparently it's cold). Yes, it's waterproof and I wear it in the bath.
I don't actually use this with a Sony Ericsson phone, I use an Android device and the OpenWatch software from the Android market. Apparently this software is also available for Windows Mobile and Blackberry devices.
From a technical standpoint the watch acts as a computer and communicates with a single Bluetooth device over an RFCOMM channel as if talking to an AT modem, it then uses the Sony Ericsson extensions to request a specific set of out-of-band event messages. The spec for this protocol is freely available from SE if you are a hacker who is excited by this sort of thing. The device itself is not programmable so if you're that way inclined, extending its functionality can only be done by abusing the built-in message types. This means that it's not possible to unlock your phone or computer via a button press, have simultaneous buttons explode a remote mine or easily make the volume buttons navigate a remote menu system.
On the whole I'm happy with my watch, if it was more hacker-friendly or came with a laser and/or garotte I'd give it 5 stars.