I had a Verizon E815 and I'm trying out the W755 now.
I'd say the voice quality is almost as good as the E815.  Not quite.
I turned off voice privacy to improve quality but still, it's not
quite right.  Little dropouts and glitches are apparent in all the
calls I've made.  Oddly this doesn't seem to be the case when I use my
Moto bluetooth headset.
I get substantial interference when using the speakerphone.  At higher
volumes I think the speaker is setting off the mic, so the speaker
goes silent - sort of a half-duplex situation.
The battery is definitely weaker.
The keypad is slower to respond, really slow when entering digits.
It has some wacky software limitations as well.  Groups cannot be
larger than 10 people?  Great for small families and people who have
no friends I guess.
Also there are only three alarm clock settings, how much memory can
those possibly take up?
Looks like the W755 is going back.
Plus, I miss my Scrabble Blast and it can neither be transferred over
(thanks BREW) nor can it be re-purchased since Jamdat got eaten by EA
games and EA won't sell it for some reason.
The E815 is big but man that thing works and you can hack it all you
want.  The W755 is so new there are few hacks available for it at this
time.