OnLine on RAID - what's the truth?

I have a client who's attempting to run OnLine 5.10 on a Compaq RAID 5 architecture. I know little about RAID (other than the basic levels and what they are designed to do), except that early implementations of RAID 5 were DISASTROUS for OnLine's performance but that newer products, with their various smart disk management capabilities, are claimed to be as quick as a raw disk-based system, provided you choose the right RAID level for each component (e.g. don't put the logs on RAID 5!) The Compaq website makes ambitious claims as to how well their RAID systems should perform with an RDBMS (of course). However, the "Informix-OnLine Dynamic Server Handbook" by Carlton Doe today states "RAID levels 5 and 6 are the slowest of all the RAID levels. You should only implement this level if access speed is not an issue ... The I/O throughput for these levels is very low because of the disk contention involved ... The one advantage these two RAID levels have is that they provide a decent degree of data availability at about half the disk cost of [RAID 1]." What's the truth? Can anyone speak from experience on this subject please? -Andy Kent- Bristol, England.