Having set up several Mac Pros with the Apple RAID card, I can say I really like it, both hardware and their software interface. Used the RAID 5 option for the best compromise of a single large share, good performance, and some drive failure protection. As I recall, last box I tested, I was getting about 400MB/s throughput with 4 1TB HDs in a single RAID 5 config. No complaints.
Having said that, I don't think the card is worth the price to a do a couple simple stripes and/or mirrors, which can be had for free via software (Disk Utility) with no performance costs, overhead, etc. Though not common, should you have a problem with a RAID 5 array, there is little chance of recovery......so be sure you backup early and often. RAIDs (all of them....) are not a backup!
If you don't want or need a RAID 5, pull the card and sell it. New they run $700ish, so ought to fetch at least half that, if not more.
"Imagine if every Thursday your shoes exploded if you tied them the usual way. This happens to us all the time with computers, and nobody thinks of complaining." -- Jef Raskin