What does the dynamic feel like between long time clients and providers ?

Im only hitting the year mark with a few regulars now and a post today about real names had me wonder - what does seeing the same client or provider for 5-10 years feel like? I'm just really curious! Do long time clients end up feeling similar to friends, or is it something else entirely?
What do you think they key is to making a dynamic work for many many years is? I'd guess you both would need to feel secure in the transaction and have lines and routines you don't blur?

If anyone feels like sharing their experiences with this, I'd love to know! I find it really interesting...