Figured I'd join in and share my horrible horrible gentoo woes...
I have the 8890, with a Promise Fasttrak100 Lite, aka LP in promise's terms. Their partial source drive is CRAP. It does not compile. So, I did some creative things to get gentoo installed on my system, but in the end, all unsuccessful.
First thing I did was install Red Hat 9. Very straightforward install, using promise's precompiled drivers. Everything was dandy, except I hated RH9 as soon as it did install. First thing, no sound. Their kernel did not have all the drivers installed for Alsa to work, and any attempts to modify the kernel resulted in that VFS no root error. Second thing, very VERY old software. KDE 3.0, not 3.2, some ancient gnome version, everything's old. Any attempts to update also resulted in that no root error. So, I gave up with RH9, and tried to install gentoo from under RH9.
RH9 could read the RAID array, so I simply made a new partition, and extracted gentoo's stage file onto it. I chrooted into that and proceeded to emerge the kernel and the partial source promise drivers. Kernel compiled properly, but booting it resulted in no root, since I don't have a raid driver. Partial source promise drivers did not compile, until I went in and edited a bit of source (for kernel version). When they did compile, I made a new initrd with mkinitrd with those drivers, along with the scsi_mod and sd drivers already in the /lib/modules directory. Booting them resulted in a kernel panic, then a VFS no root error
Then, I started doing creative things. I took the initrd from RH9 and tried to boot it with gentoo's kernel. Didn't work, because I didn't have the exact same ancient 2.4 kernel that RH9 had. So, I emerged that ancient version, and copied RH9's config file, and recompiled. Still didn't work. I dissected the precompiled initrd for RH9 and took the Fasttrak.o and put it in a new gentoo initrd, with gentoo's sd and scsi_mod drivers. I got it to say, "Promise Fasttrak 100 LP Driver, Detecting devices" or something like that, but instead of printing out partition information like Red Hat, I just skipped on and gave me the VFS no root error. I also tried using Redhat's kernel image and its own initrd to boot gentoo's partition. I managed to get into gentoo, but considering it's not built with devfsd or any of the alsa drivers, it's useless.
Also, the kernel's own raid drivers are borked with our controllers. For some VERY odd reason, using the ataraid/pdcraid modules default on the kernel allows the kernel to see the drives, but any read/write attempts are almost always corrupted. All the data shows corruption, and drives/partitions/data randomly vanish, like voodoo magic.
In other words, promise's drivers and controllers SUCK. No support for crap, their partial source drivers don't work, and Linux is essentially non-functional on their drivers. Best bet? Kill raid, and windows, and use Linux software raid. Unfortunately, windows will not be able to use the drives if you're using pure LSR. Gentoo forums also indicate that LSR is a lot faster than most hardware raid, since its algorithms are more efficient, and most hardware raids (such as ours) emulated most of the raid in software (hence the borked drivers). Second best bet, use Promise's 2.6 Open source drivers, when they're released in a few years
Well, those are my experiences with RAID and Linux. Quite a journey, and very angry that it did not work. I hope you guys have better luck with Suse...