They differentiate themselves by offering SSDs for all instance sizes + offering a very cheap level ($5). While SSDs won't improve all workloads, I think "high performance" claims compared to the typically IO-weak alternative isn't unreasonable.

Also, they're popular enough now that I'm surprised people haven't heard about them (especially after recent funding)...

Yes, at least. For one I believe all DigitalOcean boxes have SSDs. I've found their boxes much better per $ than an EC2 VM. EC2 has a much wider tool suite and is possibly better if you regularly spin machines up and down. YMMV, but for just 'a box' they compete pretty well IMHO.