I've had this phone since the beginning of May. Hardware wise its great, especially the ability to record videos in 720p HD and 8.1MP photos.
Whats not so good is the operating system it uses. I didn't really look into Symbian before buying the phone but now I wish I did. I don't really like Symbian, I wish I would have got a phone running on Android now. The latest version of the Symbian OS looks good but it doesn't seem Sony will upgrade the Vivaz to it.
Also pulling the rating down is the lack of support for the phone. There aren't many good apps for it. For example, Foursquare do not have an official Symbian app. Also, the newly released official PlayStation Network app does not support Symbian.
Overall the phone is good, I got it for free on a 15pounds a month contract that includes 50 minutes, unlimited texts and 250MB data (which is enough for me!). I can cope with the phone but look forward to upgrading it.