I've read a lot of different reports that suggest at this point in time, kernel software raid is in most cases better than controller raid.
Let me define 'most cases' for you. Linux software raid can perform better or the same if you are using raid0/raid1/raid1+0 arrays. If you are using raid5/6 arrays, the most disks are involved, the better hardware raid (those with sufficient processing power and cache - a long time ago software raid 5 beat the pants of hardware raid cards based on Intel i960 chips) will perform.
I have already posted on this and there are links to performance tests on this very subject. Let me look for the post.
The basic argument seems to be that CPU's are fast enough now that the limitation on throughput is the drive itself, and that SATA resolved the bottleneck that PATA caused with kernel raid. The arguments then go on
Complete bollocks. The bottleneck is not the drives themselves as whether it is SATA/PATA disk drive performance has not changed much which is why 15k RPM disks are still king. The bottleneck is the bus be it PCI-X or PCIe 16x/8x/4x or at least the latencies involved due to bus traffic.
to give numerous examples where a failing hardware raid controller CAUSED data loss, where a raid card died and an identical raid card had to be scrounged from eBay to even read the data on the drives, etc. - problems that apparently don't happen with kernel software raid.
Buy extra cards. Duh. Easy solution for what can be a very rare problem.