I finally tried flexraid. I like it!
The setup was different from storage spaces, which I thought was relatively easy. Flexraid (I'm doing the "tRAID" module) was also pretty easy to setup, but maybe a little more difficult than storage spaces. But, it has really great wizards/tutorials so it's not a problem.
The organization is similar to SS and any other RAID software, where you have to think about how you are going to organize your drives, folders, volumes, etc. However, the feature that made me go to Flexraid was speed. I was a tad underwhelmed by SS speeds, so that's why I'm here. Flexraid's traid (which is their real-time raid, vs their raid-f, which is some kind of scheduled raid) is faster than SS mirrored mode, which was supposed to be decent. In copying files over now, I'm ranging between 60-100 MB/s which I am pleased with. With SS mirror, I was getting 60 max, and less overall.
I also like that I am losing less space to redundancy and mirroring.
Another great feature of tRaid is that the files are intact if you grab a single drive and stick it in another computer. I don't really know how that works as far as spreading the files around, but it's nice and comforting to know that.
Before I went here, I also tried FreeNAS. However, I was immediately intimidated by the non-windows interface. So i quit fast before I spend too much time learning linux.
So far so good, liking flexraid's traid.