Thanks to Nokia & Wom World for loaning me a Nokia E66. The E66 marks an overall an improvement over the E51 - at least on paper. But then again it costs around 50% more than the E51... So far the E66 seems like a very sophisticated piece of kit, yet a doodle to use (no activation nonsense to worry about - I was surfing within minutes of unpacking). Bizarrely while the E66's battery seems to be physically bigger than the E51's, it is rated 50mAh less. We will see how well these improvements translate into real world usage and whether it is worth ponying up an extra £100 for the privilege.
The real distinction between the two are the inclusion of GPS receiver (with A-GPS support) and a better camera, packaged in a different form factor. The extra RAM is nice, but I never had a problem with RAM usage with the E51 so I doubt I will be seeing any improvements in performance. The E66 is very similar to the new E71 apart from the different form factor and battery size. Both runs on the slightly outdated Series 60 3rd Edition Feature Pack 1 platform. It would be nice to have FP2, but as it is S60 3.1 is very stable and snappy.