I'm a potential customer, I love their simple design and can't stop promoting Heroku to friends. I hope they succeed in solving those limitations and keep cost down in a reasonable range so that I can happily give them my money.
We have 10k/month coming in from our iPhone app and use Heroku + S3 for everything. It lets us focus on getting our product up to speed instead of system administration, period. If's only expensive if your time is worthless, or you really, really want to pay a sysadmin to take care of your server stuff.
Right now we're paying them $200/month and I smile every time I get the bill- it's one of my favorite expenses because of how much time it saves. When we get up to 5k, it's still a no-brainer. At 10k, it's time to start looking at Rackspace, but that's about the limit of things for us.
It works, it saves time, and it lets us focus on improving our product.