Y and I merged our finances as soon as we were married. We didn't really talk about it before but we had been together a long time (4 years) by the time we got married. Two months prior to us getting married, I added a cell phone onto my plan for him and I paid the bill. We got engaged that month so I didn't worry about him paying me back for it or anything. Before we got married, I told Y that I had to have my own savings account so if things went badly, I would have my own means to get out of the relationship. Side note--I watched my mom be in an abusive relationship for years without a way out and I vowed that I would never let the same happen to me.
Y and I were married and I moved across the country to be with him. I had banked at a regional bank that didn't have a branch where we were going to live. I closed my account and got cash and a cashier's check. When we got to our new location, I immediately wanted to deposit my money so I just deposited it in Y's account. He added me onto his checking and savings accounts and I pretty much took over the accounting. Y had never maintained a balance before me. I never did open my own accounts but I did keep a credit card in my name only with a 20K balance limit with a 0 balance for emergencies. I figured that 20K would get me a long ways away in the event I needed to run fast.
15 months after we were married, Y went to Iraq. Then I really took over our finances. Y is fine with it and he prefers it. We usually keep a $500 buffer in our checking account so we have enough to cover gas or groceries or other expenses. When it hits $500, it is like $0 for me.
Y and I have the same financial goals and we are on the same page (mostly) with finances. There have certainly been ups and downs with finances but it mostly works out. I try to update him with where we are at in checking and savings on a regular basis.