I've had so much more downtime with heroku/AWS than I ever had back when I was running my sites on slicehost.
I also feel like I've let my admin skills deteriorate because I've been dependent on heroku. Back when I was running everything myself, worst case scenario I could set up a new VPS from a backup in another datacenter. Now if heroku goes out I just have to twiddle my thumbs while I wait for updates.