I refi'ed my 30-year 4 3/8% mortgage, 6+ years in, to A 15-year 3% last August. My original lender (BofA) couldn't match rate, local credit unions couldn't match rate. First Internet Bank, who asked for a ton of income documentation (including my rental properties, making sure they were insured), won my wallet.
Whatever you do, never ever get in touch with Quicken Loans. They will call you incessantly. Or use a Google number. Funnily, it was a Quicken mailer that prompted me to think about refi.