From that post: "Don't use EC2 or friends except for testing; the virtualized I/O is too slow" kind of restricts Cassandra usage for an average Web 2.0 startup
Added: just to clarify that: when starting small the last thing you want to do is either buy expensive infrastructure or hack something cheap instead of doing it properly. If Cassandra won't perform well in the virtualized environment, it will be necessary to first use some other means of storing data and "go cassandra" later if needed.
But everything resembling a DB is slow on EC2 since I/O is slow and memory accesses are slow, so I think the problem is EC2 and not Cassandra, MySQL, ...
to clarify, Cassandra is still faster on a per-node basis than relational databases on EC2, but if you compare EC2 numbers with real hardware and wonder "why is it so slow?" that is why. :)
that said, there was a thread yesterday about Joe Stump's benchmarks showing EC2 I/O not being as bad as most people think so possibly they have made some improvements since. As j3fft mentions, Joe Stump is also in TFA talking about using Cassandra.
Added: just to clarify that: when starting small the last thing you want to do is either buy expensive infrastructure or hack something cheap instead of doing it properly. If Cassandra won't perform well in the virtualized environment, it will be necessary to first use some other means of storing data and "go cassandra" later if needed.