I don't actually think performance wise they have anything on a cluster of x86 servers. But its hard to benchmark things like that accurately.
From a developer point of view, its not a great technology stack to work on. Its old and out dated, and is pretty incompatible with other platforms. EBCDIC is a pain, 3270 terminal is a pain. Having to allocate files manually is a pain. Having actually about 7 different types of files is a pain.
Its really all a pain. Just dont go there.
Also, I found that guys who had been working on mainframes for a really long time have a pretty outdated view of what the "distributed" world has to offer. A lot of the time they dont understand what "web Scale" at the level of google, facebook et al are operating at.
As an additional funny annecdote, the I/O that a lot of people have been talking about is now conventional if high end disk array's AFAIK. But internally the disks are emulated for the OS to look like the disks it was using back in like 1975. Thats how it gets backwards compatibility, just dont change anything.