While other devices like laptops and tablets have edged onto the portable DVD player's turf, there are still some good reasons to consider one for your mobile video viewing needs.
First, you still have a lot of DVDs (go on, admit it). Second, a portable DVD player is inexpensive. Relative to a tablet, a laptop or even a netbook, portable DVD players are a bargain. They can usually be had for under $150 and you'll find plenty under $100 and even a few below $70. Good luck finding a $70 tablet.
Third, they offer better battery lives than many laptops. You can expect about six hours from many portable DVD player batteries and, unlike laptops, some of these portable players include a car adapter in the box, so you won't have to fork over extra money to purchase one. Others include car headrest mounts, so you can pacify your children during the cross-country trip and still easily pop out the player when you have to fly for business. Even if you don't have a mount out of the box, chances are you'd feel more comfortable handing over a portable DVD to the kids rather than your brand new iPad (unless you're ready to buy a new iPad).
More connectivity, more options
Technologically, portable players have made some improvements from the days when they just played DVDs. Today you'll find memory card slots (typically a slot for the popular SD card format) that allow you to view digital photos and some digital video files, and so you can listen to MP3 music. Portable DVD players won't be able to play back a lot of the high-definition videos you record with a camcorder, but they can handle standard definition fairly well, as long as it's in the MPEG-4 format. You'll also find USB ports for connecting a USB drive for similar digital media playing purposes.
Audio options have improved as well. Most higher-end portable players can decode Dolby Digital audio and incorporate two stereo headphone jacks, so you and a partner can listen in on the action without bothering those around you.
You shouldn't expect high-definition displays here, but the widescreen LCDs on today's portable DVD players are bright enough to provide a good viewing experience for standard definition fare. The size of the screen is usually a good indicator of price - larger screens up to 9-10-inches tend to command a premium, while the 7-inch or smaller range tend to mark the more budget-friendly choices.
When you return from your trip, a portable player can be connected to your TV or home entertainment center via an A/V output. Some (but not all) will include a remote control as well, to make it easier to operate a player that's connected to another display. If you want a player that is more dual purpose - capable of delivering a good experience on the home TV as well as on the road - be sure to select a model that supports virtual surround sound or Dolby Digital audio decoding. Also scout out a model that uses an S-video or Component/Digital Audio output instead of plain old A/V (this is a bit harder to find these days but still possible).
Portable Power Houses
Here's a look at some of the latest portable DVD players on the market:

The iLuv i1166 is an 8.9-inch portable DVD player with a variety of multimedia capabilities beyond disc playback. It sports a built-in iPod dock for playing your tunes through its built-in speakers or simply recharge your battery, or turn it into a digital frame via the SD card slot or USB port. Anti-skip protection ensures that DVD playback won't suffer if you encounter a few bumps in the road (or sky). The i1166 has a stand so it can be propped up for easier viewing. It can also play MPEG-4 video files stored on USB drives or SD memory card. It will set you back about $185.

The Audiovox DS9106 ($127) features a 9-inch LCD display that can swivel around to get you the best viewing angle. In addition to DVDs, it has a USB port and multi-memory card slot for viewing stills, Internet video or playing back digital music. There are a pair of stereo headphone jacks so two people can listen in and the battery lasts for six hours - enough to get you through at least one Lord of the Rings movie.

The Audiovox D9104 player has a slightly shorter battery (four hours) but a lower price tag at $96. It offers a 9-inch display with dual headphone jacks, stereo speakers, A/V output and MPEG-4 video playback.

Toshiba's SDP95S is another 9-inch player with a memory card slot (SDHC) and USB port for enjoying other digital content. The unit ships with three different power supply options: a six-hour battery, an AC adapter, and a car adapter. It also includes a remote capable of operating the player from up to 10 feet away (in the event you attach it to a TV or just want to squint at it from a distance). Built-in speakers support Dolby Digital audio decoding while a pair of stereo headphones offer a privacy option for couples. It costs $140.

If you're on a tighter budget, Coby offers a 7-inch "tablet-style" player for around $75. You won't find memory card slots, but the unit can play back photos and MP3 music files saved onto CD or DVD discs. The "tablet" design means you can sit it on your lap or mount it to your car's headrest to quiet the kids (a headrest mounting bag is included in the box). There's anti-skip buffering in addition to a Dolby Digital audio decoder and a pair of stereo headphone jacks.

Coby also offers a traditional flip-open player for $55. It has a 7-inch display, stereo speakers, a single headphone jack and anti-skip circuitry.