I've been using, testing mdadm for over two years with good results. I use raid5 in a four-drive array.
My system is a dual-core Intel E8400 that's very fast, with 4GB of RAM. The CPU usage during write-heavy array access is moderate, like 15% total.
Not knowing what your intended useage will be it is hard to say how the CPU performance might be.
Using raid5 increasing data throughput over a single drive, like one drive's performance times the number of drives minus one. For example, if one drive has a 70MB/sec throughput, than a three-drive raid5 will have 140MB/sec, if your controller bus is other than PCI. Both PCI-X and -E will give you over 350MB/sec if your array is large enough.
Let us know what you decide... many folks here have lots of mdadm experience, a very good program but one that has a steep learning curve, if you know what I mean. <smile>