Hi Brandon,
Second Rotation has been using Engineyard as our main service provider for over a year now. We currently have 4 production slices and 1 staging slice (we started off w/ 2 production, 1 staging). Overall, the experience has been positive. They really know rails well, set up our initial deploy scripts (by the master, Ezra) and have quickly helped us identify issues that made our app unstable (such as rogue mogrify processes). When you call, humans answer (or someone calls you back within a few minutes), their techs are helpful, patient and are usually good at answering your questions. On the downside, EY is pretty expensive and have had some growing pains. They've been slow to respond to tickets, have had perpetual load balancer issues on their original cluster (pretty much the only reason we've had any unscheduled downtime--it's been much better since we've moved to a new cluster). They seem to have more customers than they can handle (their techs are usually really busy) and we had to wait several weeks to upgrade to 4 production slices--in the meantime, they helped us by making our existing slices "fat". When we get hit w/ a lot of traffic, they can dynamically double our capacity almost instantly by adding more ram/cpu/mongrels. You cannot add slices on the fly--I think they currently have a lead time of 1 week (you can get them sooner but you have to pay a little extra). I'm hoping their recent funding by Benchmark Capital should help out by beefing up their hardware and staff, although they also just bumped up their new slice prices by $50/month.
We are also using Amazon S3 for our data warehouse and for other internally used apps. If you have any specific questions, let me know.
Cheers,
James