I've used the X series for years. It's the original MacBook Air in that it doesn't have a built-in CD-ROM drive. The CD drive comes either as an external like the Air or in a docking station "slice" that affixes to the bottom of the computer. Lenovo figured out well before Apple that people didn't need a CD drive most of the time. A quick overview of the lightest ThinkPads.
This is the latest form factor in the X line. With a starting price of ~US$500 it's a pretty good deal. Key feature differences are 11.6 inch display, a differently styled keyboard, less robust construction and generally a less robust feature set.
The X201 is the direct successor to the X200, X61, X60, X40 and others before that. Like the X100e, it has no CD drive built-in. The screen is slightly larger at 12.1 inches. The new X201 adds a faster processor than the X200 and an optional trackpad in addition to the classic ThinkPad pointing device. It also appears to make more efficient use of its battery, closing the gap with the X201s. The X201 can contain up to 8GB of RAM (vs 4GB for the 201s) and can come with built-in cellular modem (not an option in the X201s). The X201 and X201s were announced February 24; they weren't on the yet on the Lenovo site at launch but are now. (Why announce machines if you aren't going to make them available for sale at the same time is beyond me. Apple tries to set a good example in this regard but no one seems to understand or listen, including Lenovo.) You should be able to buy a well discounted X200 or X200s for a little while but it appears not directly from Lenovo--those units are off the site as of March 1.
X201s (pictured at right.)
The "s" variants have always featured a slower but more energy efficient processor resulting in longer battery life (again, with the new X201, the "s" advantage here is narrower). The X201s is lighter than the X201 (about 0.4 lb or 0.24 kg) because of carbon fiber top and glass fiber sides (vs magnesium alloy all around in the X201). It also has Lenovo's antiflex "roll-cage" design in the casing of the screen to reduce flexing. And it features a 1440x900 WXGA+ 12 inch screen instead of a 1280x800 WXGA 12 inch. Visually the X201 and X201s are almost identical. The feature comparison chart and detailed spec sheet says no built-in camera even though the pictures show a camera. Annoying!
The tablet version of the X201 is the heaviest device in the X series. The latest version has an enhanced multi-touch, pivoting screen which can be ordered in a "SuperBright" version for outdoor use in strong sunlight.
This machine goes directly up against competitors like the MacBook Air. A pricey unit no matter how you configure it, the X301 features a 13.3 inch screen which makes a very big difference over the X101e and X201. Unlike the smaller X series laptops, the X301 includes a built-in CD drive (also unlike the Air) but still maintains a competitive 3 lb +/- weight, depending upon battery configuration. Challenge with this machine (like the Air) is limited hard drive options. This machine needs to see a revision to include the multi-touch trackpad that the others now feature.
Not an X series, but Lenovo's next lightest ThinkPad, the T410s is essentially an X301 with a larger 14.1 inch screen and the option to choose a larger hard drive. It's surprisingly light. Quite a different beast than the other members of the T family.
Why buy any of these machines when they are generally the priciest PCs out there, often even more expensive than comparable Apple laptops? On the PC side, they're probably the best made laptops, the keyboards are second to none, and they have a classic look that in my opinion is comparable to Apple, though a completely different aesthetic.