Promising gear that showed potential for future (current) models

The good first:

This is a camera I bought for the IR and UV photography capability.  It has a removable  IR/UV blocker filter that sits between the lens and the mirror, rather than behind the mirror and close-mounted to the sensor, or even bonded to it.  This is a super-cool feature that makes it very versatile.  Most of Sigma cameras are built this way.

The camera is built like a tank.  I know it's almost a cliche' - but I have the feeling I could pound nails with it.  This is a somewhat old camera by today's standards, built in approximately 2006/2007 time frame.  Twelve years!  But - it still fetches $350 on Ebay, which probably means the IR/UV photographers are keeping the price up.

The color of photos is wonderful, thanks to the Foveon sensor, which is the second reason I bought this camera.

The mirrorbox has a viewfinder distance calibration adjustment (accessible inside the mirrorbox) - that makes mirrorbox calibration possible (if perhaps tedious) for a determined owner.  The vast majority of other manufacturer's cameras do not have an exposed mirror adjustment and must be returned to the factory or service center for calibration.

Then, the not so good stuff:

It was really hard for me to use the viewfinder in the sunlight, because IMO its opening is smallish, and it's not an EVF.  So, it takes significant light to brighten the view enough for me to be able to use focus properly.  I'm more myopic than most people, so the range of the diopter control lever under the viewfinder doesn't quite correct for my eyesight.  However; this was one of Sigma's initial cameras, and they are said to have corrected the viewfinder issue(s)  on more recent cameras (bigger now and with an EVF).

The sensor is part way between the M4/3 size and APS-C size (but closer to APS-C).  I am  used to the performance I get from APS-C.  Sigma's more recent cameras have larger sensor sizes (I think one is even an APS-H), and that would be a worthwhile improvement.

It seems to me that the camera needs to be used pretty close to ISO 100 or 200 for really decent pics, so it's more likely one will need to use a tripod or monopole with this one.

Recently, the old girl stumbled ...

If my experiences, coupled with some reading of the internet, make for an accurate  indication, then the camera has an Achilles heel in the design of the mirror spring connection on the back of the mirrorbox.  It's a simple fix that doesn't even require new parts, except that one must disassemble the camera to effect the repair, since the spring is on the other side of the mirrorbox.  Only a few non-service people have performed this feat. There is supposedly a firmware fix that runs the mirror motor backwards to reattach the spring, but I have not tried it. If I can find the firmware, I'll give that a try, and report back.  It's almost certain this issue has been remedied in later models (I would hope it has).  With an older camera (which is out of warranty) - this is kind of a show stopper.  Mine had a shutter count of 18,000 shutter releases when the spring thing happened.

Then, more good stuff:

Having this camera proved to me the value of the Foveon sensor for color rendition.  IMO some photos were "glorious" in terms of color richness. Many of the photos I took were shot as infrared photos (actually about 13,000 of 18,000 were IR), so owning the camera  proved to me just how valuable the removable filter can be.  It's very handy.  As previously mentioned, the camera is built very sturdily.  And the Foveon sensor rocks!  The ownership of this camera, in spite of some issues that I had with it, still makes me want to give one of the manufacturer's more recent models a whirl.

Because all/most-all later camera models from this company have removable IR filters as well as other advancements, I think the status of this camera and review is mostly just for the historical record, as almost everyone will want to purchase a newer camera.  I think the year 2006 is too far gone to be relevant, for most people.  But, if a real bargain  comes along, and you're into the IR or UV stuff - go for it.