Google makes every candidate sign an NDA that prevents talking about the interview questions and such. However, I can share information that is already posted in public blogs. First there are two different types of software engineers. Both do coding, design, development, research. Basically though there are those who carry a pager and are always on call, and those who don't. Having done the interview for both types of positions, I can say the general format of the interview remains the same, but there are noticable differences in the style of questions. That said it is an interview designed so a new graduate will actually have a better chance than 20 year veteran. Someone who is unemployed and can dedicate weeks preparing will do better than someone who can just squeeze in a weekend of refresher study. A CS major will do better than a physicist. Your references, experiences, etc will not aid for the interview. So even if you have several VP's as reference, you still need to pass the interview before that will taken into account.
As to what the interview feels like. Imagine you decide to sit in on a mid-level undergrate course at MIT like CS 170. The instructor recognizes you and asks you to teach the class. So you are up in front of at the white board trying to present solutions to problems you might not have ever seen before that day. Now imagine doing that 5 or 6 times in a row. That is what it feels like. When you pull it off and come-up with a good solution, it feels really nice. When you hit something where you spend most of your time just really understanding the problem and do several false starts, it feels really embaressing.
That said, with free online university courses available, and a wealth of reference materials available, I feel like this is an interview anyone can pass, if and only if they have the adequate amount of time to prepare. Finding that time can be difficult if your current job keeps you working 60+/hours a week, and your family keeps you busy for the rest. That is probably why you see many more young faces at google than older ones. It just becomes so much harder to find the time... But at least if you know what to prepare for and have some time, you have a fighting chance.