The Logitech Harmony Remote Controls have got to be some of the coolest, best TV/VCR/DVD/Audio/MMPC/X-10/XBox/etc remote control ever. They use a USB connection to the PC for programming, can learn from other remotes, support practically every device under the sun, they look sleek and elegant, and are incredibly easy to use.  A number of friends have asked me about it so here's my spiel on it.  This is definitely my favorite home theater accessory.  I call "ultimate", not because it has the most features, but because it is just so simple and yet effective.

Activity Based Workflow
Probably the single best feature, and something you really appreciate once you have it, is the beautiful simplicity of the activity workflow design. Once you program the devices you have (via a nice GUI PC app), and what you use them for (Media Center PC for DVDs, Cable box for TV, etc), the display of the remote says "What would you like to do? Watch TV, Play DVD, Listen to CDs". Just one click of a button turns on the appropriate devices, sets the right inputs, and sets the remote's buttons to control the appropriate devices for the activity. Of course, you can also select which device you specifically want to control (like a classic remote, without being in an activity), but I rarely have to do this since it's always setup up just perfectly.

Sleek Design
These remotes just look damn cool! They're not big and bulky like the RCA RCU1010, the Sony RMAV2500 or other PDA based remotes.  Just slick and elegant.

The base model, Harmony 550, is about $110 on PriceGrabber.com, and has most the features of the top models. The model I have, the Harmony 880 (pictured), has a color display and recharging base. It runs for about $160 on PriceGrabber.com.  See below for more models.

Super Simple Setup
Dang, these are easy to setup and use! Using a Windows PC App, you just select what devices you own, what you use them for, and it's downloaded to the remote via a USB cable.  They're also learning remotes, but even with all the eccentric devices I have, I've never had to use the learning part.  I have a Comcast DVR, X-10 Home Automation, Windows Media Center PC, Toshiba HDTV, and Marantz Audio Receiver.  It supports thousands of other devices.  Any of the buttons can be set up to control any device and it support macros, like it dims my living lights when I start a DVD.

Just One Remote
Literally.  You know how it goes, got half a dozen or more remotes?  Maybe you've tried a standard "Universal Remote", perhaps even a learning remote.  For X-10 Home Automation, maybe the Universal 5-in-1 Remote.  But if you're at all like me, you've got to keep the other remotes around just for those buttons the "universal remote" doesn't have, or the list of supported devices doesn't include all your hardware, or the "learning" process is anything but perfect.  All that is gone with these Harmony remotes.  Since programming my Harmony remote, I haven't had to touch one single original remote, period.  It is truly a universal remote. 

Available Models
Here are some of the better models available.  They all have backlit keys.  The 890 and 880 have color displays.  The 880 is the one I have and in my opinion, the best value for the buck.

Harmony 720
Latest model!
$180
Harmony 890
RF instead of IR
~$260
Harmony 880
My remote
~$160
Harmony 550
Great Base Model
~$110

Cons
These are easy, quick to setup remotes, but they don't support TV Listings, XML Editing, or support FireFox online programming.  There are many other remotes, typically a lot more expensive, that "do everything under the sun" w/ touch screens, high-res displays, and more (see RemoteShoppe.com).  But if you're a typical A/V user that appreciates Effective Simplicity (my life philosophy), these are for you.

Where Can I Get One?
Best Buy sells a few models (you can see it in the store), Amazon.com carries the full line, there's eBay, some of the best prices are at PriceGrabber.com or Froogle.com. I'd recommend choosing a model from Logitech's Harmony site and then use PriceGrabber.com to find the best deal.

The latest model, the Harmony 720, which looks really nice and isn't even on Logitech's website yet, is only available in a few places like Costco.com and Amazon.comCNet has a good review and I've seen it in person at Costco, looks real nice, slimmer than the 880 that I have.  The buttons are easier to see and from the reviews and the display is brighter.

All of the models operate the exact same way.  They all have basically the same buttons and functions.  The only difference between them is in perks like the RF of the 890, the color displays and built-in rechargeable batteries of the 880, 890, and 720.  Other stuff is just preferences, like the slim design of the 720 and 550 vs the rounder "fits in your hand" feel of the 890 and 880. 

References
More info...