I do know about the Sigma, but haven't actually tried one myself.
I've seen sample-photos from it and found the images very crisp looking and good looking.
I'll say it like that. After seeing what my Merrill is capable of, I have never looked at the Bayer based cameras the same way. That includes my beloved Micro 4/3 gear.
But I have also read about it being, as you describe, quirky to work with
The quirkiness comes from limited shooting envelope of Foveon sensor. You're better of sticking to base ISO and nailing your exposure. For monochrome, it's much more relaxed, since there's no color blotching to worry about. I'd say you can easily go up to ISO 1600 when shooting B&W and I'll take people's word that the noise looks like a film much more so than Bayer based cameras.
, as well as having poor battery life
Fortunately, Sigma sells those cameras with two batteries, at least since Merrill
If you only turn on the camera when shooting, you can get around 70-80 shots on one battery (at least I can). But some people claim they get only about 30. Doesn't really matter much. You will not be taking hundreds of photos with it. Not if you want to keep your sanity.
Not necessarily. You basically have two choices in regards to Foveon. Either a fixed lens compact camera (the dp series) or a DSLR (sd series). The DSLRs use Sigma's SA mount. Never had the pleasure to use any sd camera, but people still use SD9 or SD14/15 DSLRs with earlier generation of Foveon sensors.
and generally not very user-friendly.
As for the DP2 Merrill I have, it's actually a very nice camera. It is slow in operation, which is nice if you want to break away from spray and pray kind of shooting (which I did), but I imagine very frustrating if spray and pray is your thing. Very nice for more thoughtful photographic experience. Obviously, not good at all for shooting any action (hence the general belief that Sigma's are for landscape and still life shooters).
The camera is very simple and minimalistic, but I found the ergonomics and UX very good. It really is well designed, although I guess DSLR users used to dozens of buttons and dials might have trouble with the simplicity of Sigma's design. For me, it was a welcomed change from feature packed OM-D with sprawling menus and exhaustive customisation options. Sometimes less is better
And it has the single best shutter button I have ever used on any camera. Ever.
The biggest weakness of Sigma is software. The OOC JPEGs are not acceptable to me (oversharpened), but the SPP software that converts the RAW files is slow and clunky. I'm sure it's much better with older generations (the Merrill files are huuge). It is actually the main reason I'm not using it as much. I still have a bunch of unprocessed shots waiting for better times. There's some support from third parties popping up, but I have not had the chance to try it yet.
Anyway, if you ever have the chance to get a second hand dp2s or Merrill on the cheap, give it a try.