The z710i is nice. I liked the w810i, but never really was comfy with the candybar thing...and don't give a hoot about the Walkman thing, or have a real need for a multipixel camera. So, the z710i was the logical choice for me, at least if I wanted to continue my transformation into a Sony Ericsson user.
It has the same UI as the w810i, RF is almost as good, it will handle a larger-capacity Memory Stick, and has a great monochrome outside screen that displays info w/o having to open the flip. Build quality is at least as good as the w810i, with no creaks or squeaks so far. And...the best thing for me...the earpiece volume is far superior to the w810i in loudness. Call quality is also good on both ends, as it was on the w810i.
I have to admit that I miss the "coolness" of the w810i, and I might buy another one later for a toy, but the z710i is a super phone for someone like me that doesn't care about all the toys, bells & buzzers...just wants a great mobile phone, with maybe a few toys.
You will like the z710i...plus it can be flashed with the w710i software with Wotanserver if you want the Walkman capability.