I tried to convince one customer to use Heroku, and we did for a while, but they ended up paying big $$ to RackSpace. Considering increased admin/engineering time, I have little doubt that they would have been better off paying $500/month to Heroku, rather than RackSpace.

Another customer wanted to use Heroku, but we ended up building our own system using multiple EC2s - again, long term the hosting costs will be much less, but the admin/engineering costs especially during development were a large expense.

For my own web portal project? I am using AppEngine (code in JRuby+datamapper+Sinara, but I am considering a rewrite in Java because of the superior tool support).