Ah, I think I've found my people on this board! I've been milking mortgages for about 20 years now after a brief with extra payments on our first mortgage in the mid-90s. We've got a few now:
Residence: 2.625% fixed for another 30 months, then 25 years adjustable, balance $510k
Rental 1: 3.75% for another 29.5 years, balance $200k
Rental 2: 3.75% for another 29.5 years, balance $340k
We've been actively refinancing all of them all along, probably average about 1 per year. Always for very low cost up-front - never more than 12 mos ROI, to paying us immediate cash about 20% of the time. Only a couple were true "cash out" refinances, but we always take as much of a balance increase as the lender will allow while avoiding the higher rates for cash out. That money generally goes toward any real closing costs, escrow balances, a month or two of skipped payments. For us, the money has most directly gone toward holding all 3 properties, but also covering big expenses like college. We've been maxing out 401k and IRA contributions for about 10 years (plus smaller contributions before that). We've also built substantial nonretirement savings, invested in cheap funds and an overweight portion in a previous employer's stock that I've started selling off this year. We have good W2 income and are frugal in most important ways, but the flexibility and leverage from all of this cheap money has been a key to our savings growing so well. My wife was impressed when I pointed out just yesterday that for a few years now, our investment gains, not even including our contributions, have outpaced our job incomes for several years now. We've had some good fortune there, but we wouldn't have come close without the mortgage money on our side.
It looks like the refinancing bonanza days are probably behind us, but I expect at least one more mortgage in our future as we are nearly ready to RE. All 3 of our properties have appreciated significantly since our purchases - LTV well under 50% now for each. We may relocate, and I hope to be able to keep mortgage leverage for investment, whether that's rental property and/or market funds. And even if we stay put, our biggest mortgage now is a 5/1 ARM and there's a decent chance a refi in a couple of years would make sense - locking in a fixed rate and maybe paying it down to a non-jumbo balance.