If you are just striping for better disk mix performance, then you will not experience a lot of CPU usage from a software RAID. If you are mirroring for better reliability, then you will see a little CPU activity, but if you are running multiple cores, you won't notice anything.
If you need to run RAID 5, you will find that your CPU will have a lot more activity as the software RAID has to do all the XOR calculations on the CPU. If you find you need this level of data reliability, then spending the money on a hardware RAID is probably not beyond your reach.
As far as whether or not you will have a hard time recovering a software RAID, you will have just as hard a time recovering should your hardware RAID fail. But I can't even begin to tell you how much more flexible a software RAID is over a hardware RAID. Not being tied to a particular piece of hardware makes recovery that much easier, and on most servers, I have found that the CPU drag of a software RAID isn't even noticeable.
To be honest, I have had better luck recovering Linux MDDM RAID arrays than I have had with any other, software or hardware. With today's CPUs, you will actually find that for a lot of the tests, software RAIDS are competitive with hardware RAIDS, and well-written software (and frequent patches) will save you from data corruption. I have installed several hardware RAIDs from reputable companies where I actually had to go through at least four or five ROM flashes before I was able to overcome data corruption. I can't emphasize it enough...BACKUP, BACKUP, BACKUP. If it's important enough to require a RAID array that has parity or mirrors the data, back it up.
For what Kari is describing, this behavior happens usually when a RAID member loses either power or connectivity, and you will experience this discombobulation whether you are running a hardware or software RAID. If you back up religiously (like you should) it shouldn't take too long to recover the volumes. Again, not all RAID systems are alike, and you will experience different levels of recovery depending on how well the hardware or software was designed. I will reiterate, I have had the best experience recovering Linux MDDM RAIDs, even when this has happened. As far as the Nvidia RAID software is concerned, it all depends on how well it was coded. Bad code == bad recovery methods. Choose your solution wisely.
For the record, I have seen some very expensive hardware RAID arrays do this too. The difference is that a hardware array will usually recover with a reboot, even if the parity information gets whacked (as long as you haven't lost more than one member for RAID 5). Well written software RAIDs will recover in much the same way. And you aren't stuck without a means of recovery should your hardware fail. Just reinstall the array software on a new box, connect the drives, and away you go.