First a foremost, saying Opera 15 is powered by Chromium is technically incorrect. The new core engine is Blink which is a fork of WebCore (the layout engine for Webkit) under the Chromium project banner. The Blink project was a joint venture between Google and Opera Software ASA. I could spend hours talking about this and how it affects you, but I think I will save that until after I have had time to check out the new browser for myself.
While Opera 15 was basically built from the ground up using all new code, it is still Opera at its heart. The features I have come to love such as the built-in speed dailer, Opera Turbo (now Off-Road Mode), and built-in feed reader are still there. With the new layout engine, Opera should fit better with web standards to minimize page errors, display glitches, and site incompatibility.
Presto (Opera's in-house layout engine) will be retired as of the Opera 12.15 release and it will be missed. The update is not mandatory for current Opera users, but it will be in the future. The question now is will Opera use this to breath new life into a great web browser, or simply become a Chrome clone.
Full press release and details from Opera about this new browser is at the link below.