I'm currently on it now. I tried for MONTHS and months trying to gain weight, and nothing was helping. I tried other appetite stimulants such as periactin and marinol. Periactin worked well for about a week then it seemed to not work anymore. Marinol works well to eat a lot at one sitting, but it has so many side effects I could only take it once a day, and then the rest of the day I still wouldn't eat. So the last try was Megace. Honestly, this has saved me from getting a Gtube. Taking 10 mL of megace (liquid form) for about a week I was able to put on 10 lbs. In a week mind you.
I'm still on Megace, and although I don't necessarily 'feel hungry' from it, I feel like I'm much more able to stomach food, and it tastes better, and the amount that I'm able to eat is MUCH MUCH larger, versus when I don't take it. I believe it kind of works in a way that stops your normal 'I'm full' trigger response to your brain, so you can eat, but not feel full, therefore eating more.
Also, I haven't noticed any side effects, although I think it did mess up my monthly cycle -- when I started taking it I skipped a period, but then it regulated itself again. Besides that I haven't noticed anything.
I highly recommend it. For me it was the only thing that really worked.
Good Luck!