I fall into the "it depends" camp. I am assuming by "success" we are talking about financial success MMM-style.
I think if you have a spouse who isn't on the same page as you financially, this would be a huge drag on your progress. But if you have similar financial goals, a S.O. would be very helpful.
For myself, my husband and I got together in later middle-age, in our 40's. We have lived together for the last year and a half. Three months after we got an apartment together, i stumbled across this website and I have been around ever since. I began to make some changes - more conscious spending, less consumerism. I asked him to make some changes with me such as eating out less and he agreed. Actually, on the eating out point, his excellent cooking has been a huge help. We never go out for Chinese food anymore, since his versions of it are so good.
Plus, of the two of us, he loves going to Costco which I don't like - it is just too big and crowded for me. So he goes and does the shopping for us there. Of course, if I was on my own, I would just suck it up and go.
Now, if I'd gotten attached to someone who said no, he wasn't interested in coming on board in any way, I am not sure how this would have worked out. I guess I just would have kept working on it on my own or been sucked back into the consumer vortex.