I run OpenBSD on a 4/110 with pfour+bwtwo, sw, etc...
I run sw with DMA enabled because it's too slow otherwise and
the occasional data corruption isn't too bad (It comes in the form
of input/output errors, for instance, in making a kernel cc1 will give
an error /tmp/cc.ABCDEFG: Input/output error... if you try again, 
it will go..)  It's certainly annoying but I've never encountered
serious data corruption (files scrambled, corrupted filesystems, etc)
and fsck handles things fine.  The pfour
+bwtwo is a different story.  The pfour code works fine with
other combinations but apparently something neds a fix in the combination
of pfour+bwtwo (sbus bwtwo works for instance).  For the lack of a decent
console, I use a 386 with an ethernet card and NCSA telnet ;) (OpenBSD
uses some amazing system resources when it detects but can't use the fb,
and I get text scrolling at the same speed with sunos, slower then hell.
It also puts a tremendous load on the system to use the console in
this manner (while text is scrolling on the screen, everything else
almost halts)

Other then that, it works fine.  I would expect the behaviour from
NetBSD to be the same because the supporting si/sw code is the same
but I think the pfour code is different or nonexistent... 

The Input/output errors only occur with 'heavy' acvitivy (compiling
will sometimes bring upon such an error..) also, tons of errors
like sw: DMA error, csr=0x340d, reset 
occur.. usually these bring on Input/output errors (These input/output
errors also seem to affect swap usage, I get swap_clean_page failed
messages when I'm going well over my 8 megs of ram, yet the kernel
seems to detect and handle this without any trouble of my own)

just me

Peace,

Chris