I definitely regret not using all mine since living long enough to have it pay off is very unlikely. I had over 200 days of SL when I retired so using all that within the last year or two for a guy who rarely called in sick would have been tough to pull off without raising red flags with my coworkers and customers; management I could've cared less about. My advice for folks with more than a few years left is start using it now so you never build up that much in the bank. My thought at the time was to have a big SL balance to use as an insurance policy against some kind of long-term disabling injury. I didn't end up needing it thankfully, which I suppose is a good thing, but like you said, leaving money on the table hurts a little bit.
On a positive note, all those aches and pains in my shoulder, wrist, elbow, back, etc. miraculously disappeared after retirement. This surprised me since I thought those would be permanent after 30 years of repetitive motion injury. Staying off the couch, exercising daily, and staying involved in leisure activities after I retired also helps.