I have one in 12 (picked up the 20 ga conversion last week but have yet to mess with it).
It works as advertised...place the wad, crank the handle. For sure get the low powder sensor....you can use powder pretty quick and not realize you're low.
Another tip...watch the new primer come down into the shellplate. Approx 20 grains of propellant flowing through that giant flash hole will make ya remember real quick.
They're fast, but like any progressive, not the best for experimentation and load development.
I never change my loads though, so I crank out a couple bags of shot worth (600-900 rounds) and then cover it back up.
I also have a Dillon 650 and they're comparable in features and doo-dads. Most Dillon products seem to be well thought out and the SL900 is no exception.
Conversions are pricey. The website is misleading in the price....you'll actually require three parts to make the conversion as simple as they tout (conversion kit, powder system and casefeed plate for a grand total of $428 versus the $315 for the conversion kit only).
They're large...very smiliar in dimensions/footprint to my 650 with casefeeder.
All in all, I've got my money out of it and would do it again.
If you want pics or have any other questions, I'll be around.
Ed