This review is not complete, at this point, it is mostly just my notes.

CanonScan 9950F review

Hard to get 35mm strips aligned perfectly (I prefer to scan ALL of the image,
leaving a black border).

Can't scan 4x5 at 4800dpi in 48bit.  Don't know if that will be a big deal, but
I expect it won't be.  2400dpi seems to bring out all of the detail that is
actually present.  I suspect that 4800dpi only really helps when using FARE
(more data giving better results), but I'll need to test that.
    After testing, I've confirmed that 4800 does bring out slightly more detail,
    at least when scanning Fuji Astia (RVP100F).

17 photos - took hours using FARE on medium, which left strange artifacts on one
heavily scratched negative.

30x35mm @ 4800dpi takes about 5 hours - with all processing options turned off.

The color management is horrible.  When trying to apply profiles and target
color space, there is massive posterization.  "Recommended" color is
good if you aren't the type to mess around in PhotoShop.  "None" is
the best choice if you want full control.  None looks closer to the slide,
Recommended looks slightly tweaked (more saturation and contrast).

Really really annoying high pitched grating sound when scanning.  I have to use
the quiet mode unless I'm planning on leaving the room for a few hours after the
scan starts, damn the speed.

Scans take a very long time at 4800dpi.

ScanGear CS has a rather mediocre UI.  Changing one setting
"helpfully" changes other settings.

It would be nice if ScanGear CS saved the scan settings as EXIF data, which
would make it easier to determine the best settings.

It only shows 12 previews for 35mm strips of negative, even though I have room
on my monitor for 30.  Placement of the images should be in the same row
arrangement as the film holder.

annoying blinking blue light - covered with electrical tape

FARE artifacts are ugly, it is pretty much useless to me.  I can do a much
better job with the healing brush in PhotoShop, or just reclean the film and
rescan.

why not 36 35mm at a time?!?!?

Banding issue: I suspect it is because it isn't light tight.  The annoying
blinking LED probably doesn't help.  I'm thinking of covering it with a
focusing cloth when scanning so that ambient light doesn't affect the scan.

marketing slogan: 9950F, its the Holga of scanners.

probable scanning procedure: "contact sheet", low resolution using 35mm tray.
Any keepers get rescanned at high resolution using 6x6 holder (solves newton
ring problem?, does allow perfect cropping, able to even include sproket holes).
For photos at the edge, an additional piece of film is used as support. I'm
probably going to find a piece of black plastic and cut it to the exact right
size to mask it (otherwise sprocket holes can relect back onto the film).
If It wasn't for the single 6x?holder being able to do 35mm single, I probably
would have returned it.  I *never* return stuff.

very slow to respond to cancel

when in ... mode, it ignores changes to dpi settings, only applies setting to
one neg.

4x5 holder works well

120 holder is crap (can't scan 3 6x6 at a time due to curvature of the film.  It
looks like the variable size 120 holder is the way to go.

6x6 can't be scanned at 4800dpi, over 10,000 pixels wide (2.2 inches x 4800 =
10560) without cropping.

120 6byX holder - very easy to misalign holder and damage film. click stops
should be on both sides - drifts out of square easily

Kodak E100G 120 - horrendous magenta color cast, not present in slides.  It
switched back to "recommended" color, I think all by itself.  Yuck.

More than one hour --- Why not 2,3,4,5,6 etc?!?!?!  5 hours is a lot different
than an hour and a quarter!

Newton rings showed up in 3 out of 27 of my first scans, and continue to be a
problem.  Subtle rings aren't always noticeable,  but rear their ugly heads when
you start doing extreme curve/saturation adjustments.  The holders don't hold
the film flat enough, which also seems to affect sharpness.  I've even had a few
straight lines become noticeably curved.

Dust is becoming the bane of my existance.  I thought dust was a pain with
digital sensors --- it really doesn't compare to the hassles of trying to keep
film and scanner clean.

Can't use PhotoShop for anything else while waiting for scans to finish.
Fortunately, it is possible to have 2 versions of PhotoShop installed on one
computer.  I end up using CS2 to edit the files, and CS1 to do the scanning, and
check for acceptable quality.  So far, I'm getting about a 50/50 split of film
that needs to be rescanned vs kept.

35mm @4800dpi = 9 or 10 minutes, 15 with FARE on low.

approx 30MPix file for 35mm when scanned at 4800dpi.

scans not repeatable.  2 scans in a row will have differences in color, shape,
etc.

Software is very slow to start when launched from PhotoShop.

Tried flipping over negative to get rid of newtons rings.  Newton rings
dissappeared, but scan was softer, lots of small detail obscured.

The 120/220 scanning tray looks well designed.

A CD of scans from a photofinisher had better sharpening, but the files were
too small.  I'm not sure if this is a reflection on my abilities with
PhotoShop's sharpening filters, or if it is a focus issue with the CanonScan
9950F.  Compared to the photofinisher's scan, the 9950F seem to have a better
dynamic range, and are free from automated tonal adjustments.

The autocrop is stupid.  You absolutely need to crop manually, or it ignores
severely under/over exposed images.  It also misjudges the edge of the frame
about 10% of the time.

Rating: 2 out of 5 (mediocre)
Recommendation: I cannot recommend this scanner for serious use. It is acceptable for photos that will not be modified heavily, or printed at large size. Photos that will have little manipulation should be able to print acceptable at large sizes. If all you need is images suitable for the web, and you are willing to rescan images with Newton rings, it'll work for you. It would be nice to have a $10,000+ drum scanner to compare it with. Overall, I would say that the combination of 35mm film and this scanner results in quality approximately equivalent to my 10D.

links:
CanoScan 9950F official page