FileSonic is fast when its working. I can tell from personal experience that out of 100 tries I managed to get 2-3 files uploaded. But they seem to make progress the last 2-3 days. Unfortunately I cant see they fixing all the bugs in the near future, the site is pretty fucked and its been like that all the time I've been on it (approx 1 1/2 month). I think they are popular among downloaders because I post with HF - FSn - FS, in that order and I have far more HF links out but download count of FSn is not that much lower than HF. I think that if I had as many FSn links as HF they would be equal I think...
They are not scam, they pay. I wouldnt recomend them today, wait 2-3 weeks and see if they've sorted out their mess.