An all in one product can appeal to some users, and the special features can come as an extra bonus. Example, someone who has never owned a mp3 player buys a phone with an mp3 player, and then they can listen to a few tunes here and there while they are out. that does come in handy.
As for handheld/portable consoles tryying to replace other items, it's something that is beyond belief and it's jsut to much effort or some will find it at a inconvience since they paid that much extra for that feature & something else they already own and use on a more frequent or day to day basis already does that. The engage from nokia is one such example, and it hardly made any profit of that, a big marketing failure imo.
I frown upon a psp generally, why bother payying for movies that can "only" play on a psp, witch then makes it useless and that is the only use it will ever make, where as if you have a laptop you can make more use out of for playying video files. Imo the ds is also useles in some respect, I don't think i'd ever walk around with a ds and use msn when I am bored since I can do that just about anywhere, at home, at a net cafe or at a friend's place are just some examples. I'd only use the extra features in the odd case if they provide a useful service to me on something I might never really literally buy or something I don't already have, that's about it.
I see people who want music on there phone, but yet they have a stand alone music device such as an ipod or some mp3 player. Sure, it could be useful for listening to music when one of the two runs out of battery, but at times they are lsitennig to music on their phones and when they still have battery left on their music device, it really leaves me to question, is stayying in contact with people, to make use of the battery that way more important then draining the battery of a device souly used (or mostly) for stayying in contact with people via a phone call or a sms? Then you go see them ask a friend if they can use there phone (sure, could be for numerous reasons that I could list, such as no credit left, a thing I see more commonly in my day to day living) but there are also people who have run out of battery and have a phone capable of playying mp3s, and they are putting the span of being contactable when they are out at risk when they waste there battery for the purpose of listening to music.
It's also another factor, that the more the features you use, the more it's gonig to drain on the device's battery.