I've been taking Tamoxifen for a few months so I can't answer the long term questions I'm afraid 

I'm 34 and was diagnosed with advanced ovarian cancer last year. I became really ill just after diagnosis so started on tamoxifen and chemotherapy until I was strong enough for the op. 

I've had no major side effects, I think it has put me in early menopause as my period has stopped (though that could be the chemo too). I have some hot flushes during the day but they honestly aren't as bad as I was fearing. I've also have night sweats, it was bad for the first few weeks but seem to have calmed now. 

The only major complication I've had, I developed a clot in my lung but was told that this is a common side effect to the cancer/chemo/tamoxifen. I'm on blood thinners for a few months but its not affected my lifestyle in any way. 

In all, there have been some side effects but I'd have probably experienced them at some point in my cancer treatment anyway. It was given to me as a last attempt to limit the tumour growth before my op, and its done that so the benefits far outweigh the inconvenience of the side effects.