Heroku is the only business critical service I use that makes me smile almost everyday. I wish my telephone company, ISP and bank could provide such an excellent product with the same levels of customer service.
This week I finished migrating the last of my clients' applications over to Heroku from SliceHost. This has dropped the hosting fees for most of them down to zero while at the same time really impressing my clients with significant speed increases. I might have been able to get the same performance increase out of SliceHost if I was more skilled at sysadmin but I'm not a sysadmin.