A lot of guys at other hairloss sites seem convinced that it works better at regrowing hair than the liquid. Some of these guys cite increased shedding early on in its use as a sign that it is working better, some feel that their hair suffers and may not recover from the increased shed. One argument is that the foam allows for better penetration. Who knows, the difference discussed by people could be a placebo effect.
I have tried and am now a convert to the foam. It's clean, dries quickly, and doesn't make the rest of my hair greasy. Previously I used a high quality minox forumulation by Dr. Lee and was very happy with it (much better than the generic liquid Rogaine, etc.), but the foam won out this one in my case, and once my current liquid supply runs out, I'll be on the foam full time. You can get it in value packs at Costco etc. and also bulk packs online.