Compared to Leica M9

First of all we'll look at the X1 alongside it's big brother, the M9. These two cameras are very different, of course, here we're seeing how the X1 inherits much of its looks and design from the classic rangefinder.

The X1 bears a distinct family resemblance to its stablemate the M9 - sharing key design cues such as top-plate control dials (for both shutter speed and aperture), rounded edges and the steel gray paint finish. It's an undeniably pretty little thing, with its distinctly 'retro' design and two-tone finish.
The rear layout is also very similar to that of the M9 - indeed if anything, it's a bit more coherent and conventional (for example the 'set' button resides in the middle of the four-way controller, just like on almost every other camera currently made). All key shooting settings are directly accessible by the press of a button.

Compared to Sigma DP2

The most similar camera to the X1 currently available is undoubtedly the Sigma DP2. Both are unashamedly 'photographer's compacts' with the bare minimum of features, built around large sensors and high quality fixed F2.8 lenses with full manual control. You can buy a DP2, its 28mm-lensed twin the DP1, and pretty well all of their accessories (optical viewfinders, lens hoods etc) and still have change from the price of the X1.

The X1 is slightly wider-bodied than the Sigma DP2 and DP1 (by about 3/8" or 1cm), mainly due to the rounded ends. The design philosophies are chalk and cheese, with the curvaceous, dial-operated Leica standing in stark contrast to the boxy black button-driven Sigma.
The rear layouts are subtly different too - there's a similar number of buttons, but whereas the Sigma places absolutely everything under the control of the right thumb, the Leica has an SLR-style row of buttons down the left side of the LCD. The X1 uses a focusing dial similar to the DP2's which sits under your thumb on the upper right of the camera, and finds space for another vertical control dial on the rear too.

Compared to Panasonic DMC-GF1

The other cameras the X1 will be competing with are the latest compact Micro Four Thirds designs, the Panasonic GF1 and Olympus E-P1. Here we're comparing against the GF1, which offers the more similar feature set (including a pop-up flash) - indeed given Leica's long-running cooperation with Panasonic, this is the camera many people expected to tweak and release under its own brand instead of the X1. The E-P1 in contrast has no flash but includes in-body image stabilization and 'retro' styling (although a distinctly modern control layout).

The X1 is quite similar in size to the Panasonic GF1, but a little wider and lower in profile. The Leica has a slightly larger sensor, but the ace up the Panasonic's sleeve is clearly its ability to accept interchangeable lenses, including the slimline 20mm F1.7 seen here (which we like a lot).
Curiously, the X1 uses a 4:3 LCD to view images from its 3:2 sensor, while the GF1 employs a 3:2 LCD and a 4:3 sensor. The GF1's control layout is again much more 'electronic'; the X1 will appeal to those looking for a more straightforward, traditional design.

Key specifications compared

Here's the key specifications of the X1 compared to its immediate competitors and two popular 'enthusiast' small sensor compacts, the Canon Powershot G11 and Panasonic LX3 (also known as the Leica D-Lux 4). Once you take the lens into account, the X1 is rather lighter than the two Micro Four Thirds cameras, and is slimmer and easier to slip into a pocket (even using the 'pancake' primes). It's smaller and lighter than the Canon G11 too, and a hair slimmer than the DP2. In this particular field, only the LX3 is clearly more portable.

* The Panasonic DMC-GF1 and Olympus E-P1 accept interchangeable lenses based on the Micro Four Thirds standard