Review Summary:
Bottom line, the ThinkPad T420 is an excellent choice for business, home, and student use.
Pros
- Good build quality
- Excellent keyboard and trackpad/touchpad
- High-resolution 900p anti-glare screen
- 10 hours of battery life!
Cons
- Weak speakers
- Design might not appeal to all
Build and Design
The T420 looks ... pretty much like what ThinkPads have always looked like. It's a classic design that places function over form. Take the exterior plastics, for example. They have a matte texture which is scratch-resistant and does not show fingerprints/dust (as opposed to glossy plastic). The plastics are of good quality and don't rattle when tapped by a fingernail. The fit and finish is consistent all over - there are no rough edges and gaps between the various pieces are the same.
The chassis strength is excellent; T-series ThinkPads have strong internal frames that prevent flex. This is good for the internal circuit boards - the less they flex the better in the long term. The lid is securely anchored to the chassis via two metal hinges. The lid itself affords the screen good protection from bumps and pressure but lacks the magnesium alloy backing of notebooks like the Dell Latitude E6420 or 
Overall the quality and build are top notch. Keep in mind the T-series notebooks are quite popular for corporate fleet use; they must handle years of abuse from not-so-careful employees in order to provide a good return on investment for the company. They're built to last and it shows.
Upgrading the T420 is a simple task; a single cover under the chassis holds the memory and the hard drive slides out the left side of the chassis.
Ports and Features
The T420 has a solid array of input and output ports. It's also worth noting the T420 has a docking station port on its underside - Lenovo sells several different models, all offering plug-and-play compatibility and connections to many more ports. This functionality is a feature of a true business notebook; lower-end "business" notebooks do not have them, nor do consumer models. All picture descriptions are left to right.
 Front: Screen latch |  Back: IEEE-1394 mini-Firewire, USB 2.0, battery pack, AC power, cooling exhaust vent |
 Left: Cooling exhaust vent, VGA, Ethernet, DisplayPort (top), USB 2.0 (bottom) |  Right: DVD burner, ExpressCard/34 (top), media card reader (bottom), USB 2.0 (top), USB 2.0 + eSATA combo port (bottom) |
Screen and Speakers
The T420 features a 14-inch screen with an anti-glare coating. It is available in two resolutions: 720p (1366x768) and 900p (1600x900); ours has the latter and it's a good thing it does, otherwise I would complain. A 1600x900 screen has almost 30% more space compared to 1366x768 - that means less scrolling since more lines of text can be shown at once (or more detail in a high-resolution picture). It also makes it possible to use two windows side-by-side.
The screen has 15 levels of brightness and ample contrast; the infamous Command Prompt window looks black with barely a hint of gray. Viewing angles are typical for a TN-type panel like this one; fine horizontally but with significant color distortion vertically. Nearly all notebooks come with TN-type panels. The anti-glare screen coating is more evidence this notebook is designed for productivity - it's not a mirror like a glossy display and thus does not have annoying reflections. It is easy to clean as a bonus.
The T420 sports two stereo speakers on either side of the keyboard. They suffice for watching occasional audio clips but lack the volume and bass to enjoy music. I appreciate the dedicated volume control (up/down/mute) buttons above the keyboard - they're convenient.
Keyboard and Touchpad
Like the design, the T420 has a keyboard ... pretty much like ThinkPads have always had - a great one. Although Lenovo is controversially introducing "Chiclet" or "island" style keyboards on some more consumer-esque ThinkPad lines, the T-series stays true to form with a standard keyboard (maybe it should be called an "old school" keyboard since apparently "keyboard" isn't sufficient anymore).
Tactile feedback - what it feels like to press the keys - starts with the key travel, or the distance between pressed and not pressed positions. The T420's keys have precisely the right amount of travel to allow time for your finger muscles to realize a key is fully depressed; this is essential for fast and accurate typing. Additionally the spring mechanism (it's not actually a spring, but a scissor switch/rubber dome, but that's for another article) pops the keys back up immediately yet is stiff enough to let you rest your fingers on the keys.
I could talk about the keyboard all day but will restrain myself to two more points. First, the layout of the keyboard is fantastic; it has the [Fx] keys at the top separated into groups of four as they should be and the home/end/pgup/pgdn/delete keys separated into cluster. Lastly, the keyboard is quiet and subdued for use in meetings and classrooms alike - no clacking or clicking, just a nice low octave sound.
The T420 has an "UltraNav" solution which consists of the touchpad and the red rubber trackpoint in the center of the keyboard. The touchpad sports an anti-glare textured surface which is actually a bit too bumpy for my tastes, coming from a smooth touchpad. On the plus side, the rougher surface makes it highly accurate. The trackpoint is the best in the business, responsive and with an excellent dome shape. I prefer using this since it's not necessary to remove hands from the keyboard while doing so. Finally, the buttons are quiet and have enough depth to provide solid feedback. They are easy to find by feel.
Overall there are not enough good things I can say about the input devices, they are the gold standard. If you want to buy a notebook with a good keyboard/touchpad and can't see it in person, a ThinkPad might be your best bet.
