The Z600 does click when you open it.  I personally like the click, because
it's part of a positive lock that holds the phone fully open.

The real disappointment from Sony Ericsson, which at first blush looked to be
a worthy successor to the Z600, is the Z500 (a bit smaller, EDGE Class 10,
speakerphone, VGA camera, video), except NO BLUETOOTH -- whatever were they
thinking?!

The transflective display on the Z600 is actually an advanced type of TFD
(Thin Film Diode) called MD-TFD (Mobile Digital-Thin Film Diode).

<http://www.phonescoop.com/glossary/term.php?gid=59>:

  Color LCDs come in many types. STN, TFT, and TFD are several common
  technologies used. STN features low power consumption and low cost,
  at the expense of image quality. TFT features excellent image quality
  and response time, but is expensive and consumes more power. TFD
  combines the best of both.

See also
<http://www.mobile-phone-directory.org/Glossary/M/Mobile_Digital_Thin_Film_Diode.html>
and <http://www.epson.co.jp/e/newsroom/tech_news/tnl0112single.pdf>

MD-TFD technology gives the Z600 a very good display with low power
consumption, part of the reason the Z600 has better battery life than handsets
with TFT displays.

The V551 screen is very slightly brighter than the Z600, but the Z600 has
truer colors, especially in difficult lighting.

The V551 has higher resolution (176x220 versus 128x160), but the Z600 has a
larger display, and in practice the difference in sharpness isn't noticeable.

Text on the V551 display is much smaller, so more fits on a given screen, but
as a result is much harder to read than the Z600, particularly in difficult
lighting.

The V551 display drains the battery much faster than the Z600 display, part of
the reason the V551 has much less battery life in use than the Z600.  (Poor
Bluetooth and EDGE power management are other factors.)

Overall, although the differences are small, I prefer the display on the Z600
to the display on the V551.  (I have both phones, so this is based on actual
experience.)

The camera on the Z600 is unquestionably mediocre and not as good as the
camera in the V551.  But I personally don't care about (or even want) a camera
in my phone -- some places prohibit phones with cameras, and when I want to
take pictures I use a real digital camera (Olympus D-40Z) that blows away any
camera phone.

(What I'd really like is a handset designed to work with real digital cameras,
and vice versa, so I could easily use the phone to send pictures taken with
the real digital camera.  Sony, with both products, is really missing the boat
here IMHO.)

For me at least the lack of Bluetooth (and even IR) is a fatal flaw.  WHATEVER
WERE THEY THINKING?!

Proof?  It's still shown as a current product on the Sony Ericsson website.