While there's a definite cachet to owning a little portable DVD player -- you can bring it out at social gatherings, or on long car trips, and be easily entertained, there's something to be said for using a notebook to replicate the DVD watching experience. It's not so easy as saying "Yeah, notebooks are great -- everyone should buy one of those", however.
Bigger screen experience
The majority of portable DVD players sport quite small LCD displays, with a quality that can vary widely. There's not much comparison between that and a 15" LCD screen when you're watching the latest big screen epic. Likewise, while the speakers on most portable DVD players aren't much to write home about, stiff competition in the multimedia notebook space has seen plenty of units hit the market with impressive speaker offerings, including units with real name-brand speakers and sub-woofers.
Your average laptop isn't just a DVD player -- it can also be a mean lean office machine, and, internal components allowing, even a decent games machine. That's pretty obvious, but the other multimedia area that most laptops can extend to -- and that most portable DVD players can't -- is extend their multimedia chops by loading additional multimedia format codecs, thus enabling playback of additional file types.
"One less gadget" Syndrome
If you know you're likely to take your laptop away on holiday with you, we'd suggest that you need a proper break, and that someone in your immediate circle of relatives ought to hide your notebook from you. If you're persistent, however (or just good at finding hidden laptops), then once you've packed the laptop, power supply, mouse, webcam, USB memory drives and a few choice DVDs and CDs, the last thing you want to do is pack yet another gadget just to watch DVDs. Having your DVD playback embedded into your existing gear will save you valuable bag space, and reduce the number of items you might accidentally leave in a hotel room.
While notebook batteries have undoubtedly gotten better over the years, your average notebook is busy burning power in a number of ways that are completely unconnected to your DVD watching experience -- running your OS, managing the speed of your processor and keeping virus and security scans going, for example. Unless you've got a reasonably high-end unit, you may not have enough juice to make it through longer films, and as you'll presumably be using the notebook for a variety of uses -- not just DVD watching -- it's fair to estimate that the lithium ion battery powering it will lose overall charging capacity quite a bit faster than a comparable portable DVD player. The practical upshot of this is that after only maybe a year to 18 months, you may find that your full charge lasts you for much less movie watching time than it used to.
Bigger isn't always better
While you benefit from the larger screen size of your laptop while watching movies, the flipside is that you're normally carrying around a much larger unit with you, adding drag to your bag and turning your shoulder into a painful and crooked travesty of its normal operation.
Probably the most obvious problem with using a laptop as a portable DVD player is the cost of a decent unit. Yes, the lower end units do sport DVD-ROM drives (or can include them for an often nominal additional cost), but those lower end units often have poor displays and even worse battery life. To get the full portability and solid battery life needed for a really mobile DVD solution, you're going to need to plump for a decent ultracompact notebook -- and there you're talking thousands of dollars, rather than the hundreds that a portable DVD player may set you back.
Portable DVD players are one-trick ponies, but they're engineered with that one purpose in mind. This means that they've (normally) got oodles of AV connectivity coming out of every socket and side. This can include several ways to hook up to TVs and stereo systems, as well as multiple headphone sockets so that additional people can enjoy their films on the go. Compare that to even some mid-range laptops, which may only sport a single headphone socket and external VGA connector, and you're talking a whole lot less in terms of external connectivity.
Ultimately the choice is yours, and, of course, if you're looking to buy a decent multimedia laptop -- or already own one -- then the ability to watch movies on the move is just another nice added bonus.