I have looked into these myself when I suffered from an illness which left me with the appetite to barely eat two meals per day.
The reviews were very hit and miss with supplements like and so I decided not to go for either.
Recently the thing that increased my appetite more than anything is . For the people it increases appetite on it is very noticeable and at times it was almost too strong for me. There are a few people who don't get the appetite increase. I wonder if it effected me more as my appetite is a little lower than some people's.
In the interim use liquid shakes in between your major meals to top the calories and the protein totals up.