The E66 has all the same features and functions that the E71 has, it just comes packaged in different skin. The changes compared to the E65 are fairly subtle. The overall footprint and basic appearance are the same. This slider phone is long and narrow. You can easily wrap your hand around it, though it will be sticking out at both ends. The materials are nice and classy, and definitely say "business" more than "fashion".
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The E66 has a large screen, with a nicely-sized set of function buttons below it. Though the D-pad was identical to the E71's, the function keys surrounding the D-pad were much larger, and this goes a long way to improving usability. They were easy to find and push, with lots of good travel and feedback.
The slider mechanism felt very solid and strong. We're not sure if this was a pre-production unit or not, but it felt final to us. The quality of the slider was top-notch. With the slider popped up, you can access the keypad. These buttons are nicely sized and have excellent travel and feedback.
The camera didn't react quite as fast as that of the E71, but it was still fast enough. It fired the flash when appropriate, and used the typical S60 tools for editing and manipulating photos after they are done.
In summary, the E66 is a powerful business-class device that shuns the QWERTY look in favor of the slider form factor. If you need the powerful capabilities of a smartphone, but not necessarily the looks, the E66 should be in the mix of devices that you consider.
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