Here's a head-to-head review of the Lenovo U400 versus the 11" basic Macbook Air.  Although they are somewhat different laptops, they both (if generous) fit the Ultrabook category and were both exactly the same price this month at Best Buy.

Price:
Macbook Air - Regular price is $999 but was on sale for $899
Lenovo U400 - Regular price is $899
Comments: If we applied the same discount to both laptops, Lenovo wins.
Decision: Lenovo U400

Screen:
Macbook Air - size 11", resolution 1366x768
Lenovo U400 - size 14", resolution 1366x768
Comments: 14" is as big as I would want to go for that resolution, otherwise the pixels are too big.  But at 14" it's still pleasant.  Aside from the smaller pixels, the colors seemed more vibrant on the macbook.
Decision: Macbook Air

Ports
Macbook Air - 1 headphone+mic, 2 USB and 1 Thunderbolt
Lenovo U400 - 1 headphone+mic, 3 USB (1xUSB 3.0), 1 HDMI, 1 Ethernet (RJ45)
Comments: Although the Thunderbolt is supposedly a universal port, it can be hard to find the adaptor you need for the connection you want.  Much nicer to just have the desired connection built in.
Decision: Lenovo U400

Keyboard
Macbook Air - Cushy soft backlight keyboard.  Easy to type on.  Great key layout.
Lenovo U400 - Harder plastic keys.  No backlight.  Terrible key layout.
Comments: My backspace usage is easily tenfold on the Lenovo vs the Mac.  This one aspect was nearly enough to turn me away but most of the other Windows laptops had the same crazy layout.
Decision: Macbook Air

Trackpad
Macbook Air - Great detection.  Intuitive gestures.  Large surface.
Lenovo U400 - Large surface.  Supplied driver painfully inadequate.  This Dell driver at least gives you 2-finger scrolling and 2 finger right click:  http://darkshade.homeip.net/stuff/R313059.exe
Comments: I even overheard sales people at Best Buy admitting nobody could match the quality of the Apple trackpad
Decision: Macbook Air

CPU
Macbook Air - Intel i5 1.6 Ghz Ultra Low Voltage
Lenovo U400 - Intel i5 2.4 Ghz full power model
Decision: Lenovo U400

Battery
Macbook Air - 4-6 hrs moderate usage
Lenovo U400 - 4-6 hrs moderate usage
Decision: Tie

Portability
Macbook Air - only slightly longer, thicker, and heavier than an iPad
Lenovo U400 - Smaller than a 15" laptop, and slightly heavier than the maximum weight to be properly classed an Ultrabook
Decision: Macbook Air

Graphics
Macbook Air - Integrated Intel graphics.  Remarkably good.  Plays WoW and Civ V well at modest settings.
Lenovo U400 - Integrated Intel graphics for 2D, discrete 1G Radeon for 3D.  Plays WoW and CoH better than my iMac (which also uses a Radeon card)
Decision: Lenovo U400

Heat
Macbook Air - Air is vented out the hinges to the display.  Heats up fast to uncomfortable levels playing games
Lenovo U400 - Air is sucked in through the keyboard and vents out the side.  Never seems to warm up to uncomfortable levels, even when pegged at 100% CPU for over an hour.
Comments: Both laptops don't use venting on the bottom, which is good because a laptop shouldn't require a special surface to sit upon.  Just my opinion.
Decision: Lenovo U400

Storage
Macbook Air - 64GB SSD
Lenovo U400 - 500GB HD, 8x Dual Layer DVD RW
Comments: Although the SSD is fast (super fast!) the amount of space disappears quickly.  15GB for the OS, 18 for WoW, 12 for CoH, 8 for Civ V, and you've run out of space with only 3 games!
Decision: Lenovo U400

Software
Macbook Air - Comes with OS X Lion plus iLife.
Lenovo U400 - Comes with Windows 7 and a modest amount of Bloatware
Comments: I'm not going to get into a war on which OS is better.  OS X is more slick, Windows 7 is not a "Walled Garden".  The whole reason I wanted to get a laptop was because I had to use software not supported by the version of OS X which will run on my old laptop.  I *could* install Windows 7 on the Air, but there wasn't enough room to have both side by side, and responsiveness and battery life are significantly worse when running Win 7.  Plus then I'd have to pay for a license of Windows.
Rant: Neither laptop actually comes with installation media!  The Air has a special recovery boot option that downloads and installs the OS over the internet.  It used to have a USB stick, but they got rid of that.  For the Lenovo, luckily, I had my own Win7 DVD I could use (I like to avoid the bloat with a fresh install).
Decision: Tie

Aesthetics:
Macbook Air - Nearly flat.  Brushed aluminum metal shell.  Solid.
Lenovo U400 - Almost as thick as a normal laptop.  Classy design.  Solid.
Comments: I'm including this category because aesthetics played into my reason for completely dismissing another laptop, which had a cheap plastic feel and looked as ugly as most (with dumb stickers plastered all over the palm rest, weirdly asymmetrical design, etc).
Decision: Macbook Air (although both are better than most)

Score:
Macbook Air - 5
Lenovo U400 - 6

Winner: Lenovo U400

Summary: Although the Macbook Air is more enjoyable to use and carry around, the Lenovo is just more practical, and in the end, that's how I decided between them.