Excellent results, after you spend 15 mins getting familiar with it..
Once you get the hang of using this program, it's really easy, and produces excellent results. I didn't use the manual much, but I'm sure it will explain things easily. You can fiddle around with the settings and create a small or big mosaic depending on the resolution you want finally. I was desperately searching for a free mosaic maker, and Andreamosaic came up aces!
Pros: Quality of the final picture; Can be used to make life-size posters
It's free!
Cons: Interface could be a bit more user-friendly, but I'm nitpicking