Hi Adam,
I have one, I also have an Edge 705 which has proper mapping and auto routing built in. I'm a bit of an evangelist for these devices, so pardon my enthusiasm for them. When I bought my first 305, it completely revolutionised my cycling, I was able to download tried and tested routes from other riders and it completely opened up hitherto unknown routes to me. Being able to upload your ride to your PC when you get home is cool, too. I know, for instance, what my fastest time is for any hill that I've ridden in the past 2 years, or the average heart rate for any given ride or part thereof. I can also look back and see when was the last time I completed a certain route, or totalled a certain distance in a given period (day, week, month, year). All this without actually having to put pen to paper or to press any buttons, you just attach the device to your bars and you're off. The stats are a tremendous motivator to get out and ride, I'm currently trying to beat last May's monthly mileage.
See, I love cycling and I also love gadgets, and this device merges my two interests
Sadly, the 305 is strictly a training device, so it's not going to give you OS coordinates. In fact, it's difficult to even get your lat/long position from it, but the workaround is to 'mark your position' which reveals the current lat/long. The 705 gives out lat/long much more easily.
Hope this helps,
Dave.