According to Sigma, the Sigma SD1 Merrill is 'for photographers who refuse to compromise'. We'd call that poetic license. You compromise on live view and video - standard features on the majority of DSLRs. You have to make allowances when it comes to shooting action, thanks to the achingly slow write times for the best quality files. You need to think twice before stretching the camera to ISO 1600 or above.
However, if you're planning on doing lots of slow-paced, methodical photography in good light, where frame rates and high ISO performance don't matter, then the Sigma SD1 Merrill represents a very fine offering indeed. It comes with a very capable sensor that produces some fantastic images.
The camera delivers on its picture quality promise at low ISO settings and, despite its lower 'true' resolution, it has the potential to outshine cameras with bigger sensors.
Images may be sharp, but the performance in other areas isn't. Picture noise debuts at lower ISO settings than we'd like, and the slow processing/write times can be crippling. There's no live view either, so if you're the kind of photographer who relies on this function for magnified manual focus, then you're going to have to look elsewhere.
In terms of specification, the Sigma SD1 Merrill might lack the finery of other cameras at this price point, but its stripped back set of controls is refreshing.
Its limitations do mean that this isn't a camera we can recommend as an all-rounder, though. If you're after a DSLR that offers high resolution, high speed and high-spec features, look at the Canon 5D Mark III or Sony Alpha A77 instead. This way, you'll also be able to use Sigma lenses without being restricted to a camera that just has a Sigma mount.
Both delightfully simple and deeply frustrating, the Sigma SD1 Merrill is the Jeckyll and Hyde of DSLRs. For photography in good light at low ISOs it's absolutely terrific. For everything else, it's just OK.