This question hits close to home for me.
My boyfriend and I met in graduate school, where we were neighbors and had a lot of free time to hang out. After graduation we moved to the same city. Mindful of how different our lives would be doing the 9-5, we decided to live separately to make sure our relationship could make it in the context of real world pressures. (Also my mom is religious, and I was raised not to live with someone before you were married). I moved in with a roommate to save money, he moved into an apartment with his brother, who was temporarily also in the city. We lived about 10-15 minute walk from each other. It had its challenges. I never really felt like my apartment was my own, due to an awful / territorial roommate. My boyfriend was less comfortable coming over there. We had a terrible winter, and trudging back and forth through the snow got a little old. So it wasn't ideal, mostly in that I was staying over at his place a lot more and not feeling very grounded in a new city.
When my roommate unilaterally invited her S.O. to move in (rent free!) without discussing it with me, I was able to get out of the lease without penalty. Boyfriend's bro had moved out by that time, so I moved in with him "temporarily". We called it summer camp to underscore its temporary nature; we just didn't want to scale up / pressure our relationship before it was time. Also, we have some pretty divergent interests (reading/art v. tv/sports type of dichotomy), and felt like it would be better to have our own spaces to cultivate. He made me totally welcome in his home, but it was still his home, and it felt distressing after a while not having my own space, and feeling like my stuff was just kind of layered on top of his. When I saw an apartment in an extremely close, and mostly owner-occupied apartment complex, I snatched it up and moved out. I told everyone we might just be one of those couples who preferred separate space. (NYT had an article on this not long ago; I think Tim Burton and Helena Bonham Carter are a prominent couple who lives next door to each other). I spent a good amount of money, and we both spent a lot of time getting things set up, although the same thing happened - his space was more established than mine was so we ended up cooking there, sleeping there pretty often out of habit and preference.
Within a few months of moving into my space I found MMM. It didn't really change my concept of our living spaces until I realized that my boyfriend's lease was ending in one month. I had a eureka moment and suggested he move in with me - we would save thousands of dollars in rent, we hung out / slept together all the time anyway, and we might as well figure out how to blend now, rather than later. Also, I was sick of not feeling rooted in a space. He was opposed as he loves his apartment (it is gorgeous), and it was such a lightning bolt / last minute idea. But pandora's box opened, and it all of a sudden seemed like all the original arguments for living separately didn't carry much water. We figured that even with a 5 minute walk between our apartments, we would still end up walking back and forth to each other's places over 40-hours a year + wasting scads of cash on double rent and utilities. We ultimately decided that I should move back into his apartment, but this time we would re-arrange things and make it "our apartment". And we did. The two of us combined in that space makes amazing use of it, and we both feel at home. However, I only feel like this worked out because of who my boyfriend and I are, and our compatibility. I think the cost of living together is emotionally too high, even given savings, if it is the wrong guy.
TLDR: After 3 years together we still had lots of excuses not to live together (not wanting to rush / compatibility fears / tradition / desire for independence); but putting it off was expensive in both time & $, despite being practically neighbors. Living together is fantastic, it worked out well for us.
libraryjoy, what is your situation?