I have experience taking Cymbalta for nearly two years, but it was prescribed to help with anxiety issues, not fibromyalgia (I have both).
I was working as an administrator at a behavioral health facility (mental health), so when I spoke with our medical director about some issues I was having that I believed were depression related, he told me that he believed that it was anxiety related rather than depression. He prescribed another antidepressant at first (Effexor), but due to side effects, he switched me to Cymbalta - which was purported to have less side effects. (I've always been really sensitive to meds... lots of allergies to them and adverse reactions.)
At the time I was taking Cymbalta, I also had fibromyalgia, but it had not yet been officially diagnosed. That came several years later by a rheumatologist when my primary care doctor sent me there to rule out RA or lupus, due to the increased pain I was having. The Cymbalta did help with the mood disorder I was dealing with, it settled me to a more even keel - less anxiety and depressive feelings. However, ultimately, after taking it for a year or so, I realized that it was almost TOO successful in evening out my moods. Although I wasn't feeling anxiety or sadness like I had been, I was also not feeling much of anything else. I felt "flat." Technically, it was called emotional blunting. I was losing all my highs and lows. I didn't get very happy or sad about anything. I found it to be very disconcerting. I mentioned it to a social worker on staff who said she'd taken Cymbalta and had the same reaction, and she told me she'd ultimately discontinued taking it.
When I mentioned it to my boss, he lowered the dose a little. As I said, I'm "sensitive," and it doesn't take much for me in the way of dosages of anything. My dentist used to joke that he had to give me a "toddler dose" when he gave me nitrous oxide gas, or it nearly knocked me totally out. So take my whole experience with a grain of salt - you may not have the kind of sensitivity to Cymbalta that I had.
Ultimately after a year or so I wanted to discontinue taking it. I'd been getting other nagging side effects (including brain zaps - a phenomenon you might want to Google), and I just felt it wasn't worth it anymore. My boss told me I needed to wean, wean, wean off of it - not to stop abruptly. He said it might take a couple of months of cutting it back before getting totally off of it. In the end, it took me nine months to wean off totally, because the withdrawal syndrome was difficult.
Don't get me wrong, it's NOT an addictive medication, but after a couple of years my brain was used to what it was doing up there, and it took time for it to get used to the idea of no more Cymbalta. It was a dependency, which isn't uncommon with many meds. To be honest, I could have gone on longer than 9 months with weaning, but I was just sick of it. Since I was getting mild withdrawal issues all through the wean anyway, I figured I might as well speed up the process and get it over with, and so I did. The "brain zaps," (not dangerous, just creepy) happened now and then for at least a year after I was totally off of Cymbalta.
My experience was not uncommon. Not dangerous, just unpleasant. I did the wean following suggestions online on how others did it, which mostly consists of opening the capsule and literally counting out the tiny pills inside to cut back very slowly. On the other hand, I have a relative who also took Cymbalta (for depression) and he didn't wean off of it - he stopped cold turkey. He said he did have unpleasant withdrawal effects, but he just pushed through it because he wasn't willing to do a lengthy wean.
I'm only telling you this because you asked to hear the bad with the good. What I can tell you is that my fibromyalgia symptoms (which as I said I didn't know was fibromyalgia at the time) were not affected in any way by the Cymbalta. Not for better nor for worse. No effect on the pain whatsoever. But considering that there were side effects while I was taking it, and withdrawal effects for a long time during and after the wean, I would never be willing to take it again. Should I have depression or anxiety issues again, I'd try to avoid SNRIs altogether. I'd already had a bad reaction to Zoloft (an SSRI), and SNRIs like Cymbalta have less side effects than SSRIs for some people, but it wasn't true for me.
I really do hope that Cymbalta helps with your fibro pain, but if it doesn't after a couple of months, I wouldn't keep taking it for the heck of it.