Downloaded Technical Preview. Loved it immediately because it was back to the Opera that I knew with some changes most looking for the better. Browsing with multiple tabs about 4 (Slashdot.org, beta.Slashdot.org, Facebook and a Google Search page) when the Vivaldi browser suddenly quit. Refused to start despite several clicks on the icon. Had to restart the computer to get it back. Computer is an AMD Phenom FX-5000 Quad-Core on 4Gb RAM running Win 7 Ultimate 64-bit.
Have the same experience, I noticed it crashes after a site asked for geolocation.
You need to kill all Vivaldi processes to start it again without restarting Windows btw.