We have always had separate accounts and we're married over 18 years now.
We got a joint account when we were building an extension so drawing down mortgage payments in dribs and drabs, and those cheques needing to be cashed. Other than lodging occasional cheques from DM (who used to still send cheques to BiddyPop DHsurname until about 2 years ago - when she finally accepted that I was still BiddyPop BiddyPopSurname) when she would send a gift or repay something I'd spent for her, the only thing we use that account for is to hold some savings, as it's handy to have 1 account in joint names.
But we both work FT, and we both have to travel so need access to funds without impacting on the other.
DH pays the mortgage and his car insurance. I pay most of the other bills (although, he took over the telephone/broadband/tv a few years ago when we switched provider as he did that thankless task!). So we're happy with how it evens out.
We both add to savings, and we both have a document somewhere that the other knows about to say what accounts we have etc for if anything ever happens and we need to know.
If either of us is short for whatever reason, the other just gives money without a problem. I tend to be the one with more everyday ready cash (I prefer to keep a certain amount of cash in my wallet) while DH tends to be the one to cover big ticket expenses (better paid and better at saving). So it works out for us.
But as others have said, we trust each other and, while we don't generally know exactly how much is in the others' accounts, we have a fair idea of roughly what it should look like and the general amount of our income and the limits to what we can spend. And we do talk about any big items we want to spend before going out and spending on them.
But it also gives the freedom to have a bad day at work and go splurge on 2 lush bath bombs and 3 trashy magazines/novels to read in those baths without getting eyebrows raised at "what on earth are you buying"!!
Like I don't see his occasional wandering into tool shops and picking up yet more gadgets that won't get all that much use... So we are both happy in that ignorance. But both knowing that all the bills will get paid.