I'm running matrixed arrays of RAID 0 (4 disks) and RAID 5 (same 4 disks) through a D955XBK. The RAID 0 works just fine - no noticeable CPU overhead and pretty darn fast. The RAID 5 does not incur a lot of CPU overhead, but does have very slow write speed. This is because for every write, it actually needs to do a read-merge-write operation. Intel calls their onboard RAID 5 implementation "intelligent RAID". I call it slow - it's slower than 20 Mbps write speed and is inadequate for recording live TV shows. It's great however for anything that's write-once, read many, such as your picture collection that you don't want to lose. Read speed is also decent (because it's basically a 3-disk RAID 0) which makes it a good idea for the O/S as well. My HTPC boots a lot faster than any single-disk systems I work with these days. You just don't want to put your pagefile, temp space, or anything like that on RAID 5 so careful configuration is important.
I had a HDD fail recently I was able to rebuild the RAID 5 array very easily. Even doing a RAID 5 rebuild, the CPU usage stays pretty low. If I use a different disk for temp storage (I have 6 installed), I can even watch and record TV on MCE during the rebuild with no lost frames.
I used to have a RAID 10 (01?) array based on the same Mobo and disks, but the performance of that wasn't great, and the storage penalty was too high.
If you have only two disks, RAID 5 isn't even an option. In that situation, I will say that neither RAID 0 nor RAID 1 should incur a big CPU hit because neither require the costly XOR operation that RAID 5 uses.
Update: After fixing a problem with a driver, I have observed up to 5 MB / s (40 mbps, twice the previous stated number) write performance with the RAID 5 array.