Overall, my experience was extremely positive. I had the opportunity to interview with the team l'd be working directly with, so it was nice to get to know the team and get a good sense of the responsibilities I'd be handling.
You'll be asked questions about your experience with Adwords and any relevant experience you bring to the role. You may be asked a few hypotheticals, but all very straightforward.
In terms of what to expect, there's a phone screen with the recruiter, followed by a call with the hiring manager. If those go well, you'll move on to onsite interviews where you'll meet with a few folks from your team and maybe 1-2 non-team members who'll ask general questions to form an objective opinion. You may have followup interviews, which could be in-person or through Google Hangouts.
I think the biggest thing to bear in mind during onsites is that your interviewers may be taking notes the whole time, likely on their laptop. Don't let it throw you, they have to do this. It can be kind of off-putting, but it's totally normal for a Google interview.
On the whole, I enjoyed my interview experience and was thrilled to receive an offer! The one watchout I'd give is that this is not a quick process. That was probably the most difficult part. End-to-end, it took around 11 weeks from recruiter first email to offer.