I like how it can be used to create mobile apps without having iOS or Android development skills required (at least for some of them). So far we have only started using XaMoM but what i liked most was its ability run both desktop & web applications from one code base which makes managing our server less complex than before as well running all versions at once with just 1 app engine account made things easier! Not many downsides so no problems yet either :) However there are few minor issues such us need more documentation about deploying existing templates/widgets etc into new projects when you use custom domain names instead traditional google domains e-mails not working properly after signing up - this could probably help if they fix these 2 points first time right away :D.