I have been taking it for about a year. I know it's mostly prescribed for OAB/urge incontinence. My main problem isn't OAB, but stress incontinence due to intrinsic sphincter deficiency, which means my internal sphincter isn't closing completely (in my case due to surgery) and consequently I have a small, but constant, leakage of urine which does increase during times of movements like standing, bending, lifting, etc.
My urologist ran some test and determined I was having mild bladder spasms, which internally were forcing urine past my defective sphincter. He prescribed mirabegron (brand name Myrbetriq) to reduce/eliminate these spasms. I haven't had any follow-up test to measure the reduction in the spasms, but my rate of urine leakage is definitely reduced. Also, I have had no side-effects at all.
I have been dealing with my incontinence for about three years. I have tried several different medications. Most of them have had a positive effect on my problem, but none of them have been a "cure".
Hope this info helps