This world is not going to survive if everybody trusts what Google finds
There is nothing wrong with the Supermicro blades. All blades, from all manufacturers are proprietary -- if you buy IBM blades, you buy IBM enclosures etc.
In all cases, you need to check the specifications of the blade processors for support. With the example Supermicro, where I have researched recently for a new system build, their current blades have either and Intel disk controller, or an AMD disk controller, or an LSISAS2008 SAS controller (for blade models with more disks). There is perfect support for the Intel/AMD SATA controllers. There is currently no support for this SAS controller in 8-stable (FreeBSD up to 8.2). There is however support in FreeBSD 9 --- one could expect it will merge to 8-stable after 8.2 is released. Slightly older models of the blades have the LSISAS1068E SAS controller. All blades feature integrated dual Gigabit controllers from Intel, that too, have very good support in FreeBSD. Some blades can have Infiniband/10Gbit Ethernet controler as well. The 10Gbit part is too, supported by FreeBSD, Infiniband is not. But then, FreeBSD does not yet support Infiniband in any platform. What else is there in the blade? CPU, RAM... these are obviously supported by FreeBSD. The chasis itself has nothing to do with the operating system.
I would expect the same for the blade systems supplied by IBM, HP etc.
You ask about performance, but what about price? Supermicro 4xCPU AMD blade, with 4x2.5" drives costs around $1000 (blade only, no CPU and RAM).
This gives you up to 48 CPU cores in one blade. You may have up to 10 such blades in 7" chasis.
