Mdraid has proven quite stable for a couple of years now and shows quite trouble-free handling.
The main reliability problem you might run into is rebooting with a failed primary boot drive.
- you need to have your boot loader redundantly written to the second disk as well - a fact many admins forget
- the BIOS of your server machine needs to be able to do a failover to the second hard disk for booting, should the first fail
- you might see a scenario where a drive is not quite dead but taking ages for reading even a single sector - thus giving you a reboot taking a whole weekend.
All of these problems are alleviated by using a hardware RAID controller as it includes the necessary abstractions and fault detection logic.
Apart from that, you should be fine with a software RAID, it works quite well performance-wise too. If you need more detailed performance considerations, read the numerous performance reviews you can find on Google or in this article by Jeff Garzik - a Kernel developer at RedHat.
As for your second question, I have not much experience with Supermicro gear, but what I have seen so far was mainly commodity desktop-grade stuff built into cheap 19" enclosures. It might work out well as long as there is redundancy, I also know that large hosting providers like Strato (Germany) were using Supermicro stuff in their datacenters, but personally I would rather look for "proper" blades.