Been sitting here for the past couple minutes since you and I started communicating (thanks for the private PM) looking at the advantages/disadvantages of getting a larger, extended battery.
I know the Hyperion battery is not available in the UK, but this should apply to you as well since you're considering a larger battery...
The Hyperion battery adds bulk. That worries me. What worries me more is that if you want a case for your phone -- which would most likely be the Hyperion Case -- it's not a very good one.
I just have a hard time sacrificing the slim form factor of this form to add bulk for an extended battery.
I would much rather just buy a second battery and charger and have a spare on me at all times.
....however, that doesn't take away from the fact that this should not be even something we should be discussing. Why the #1 posted problem on this forum concerns battery life is beyond me. I love this phone -- much more than the iPhone -- but the biggest problem it has is crappy battery life.