I use a Garmin 510, and I think it is the best thing ever. No issues with running out, and the route planning is great.
The only reservations that I have are:
1. When using a programmed route, it is tricky to follow all the other information, such as speed etc, as you do not get notification when you need to turn, it just tells you when you are off course (unless I am missing something!!)
2. If you get lost, you are still buggered, as it has no maps, so you do still need to know where you are going.
The more expensive Garmins do have maps, but if I get lost, I stop and use the phone to find out where I am and get back on track, usually though I plan my route using Bikehike.co.uk, and remember it the old fashioned way!!
BTW
tooncheese did you see any of the race today? I have just watched the highlights, and they flew up Ditchling beacon. I have done that climb twice on the London to Brighton, and it is a real leg breaker!