Fakes are easy enough to pick out, and DealExtreme is good about replacing them for free, so that probably shouldn't be your main concern.
Is it worth it, though? Depends what your priorities are. As far as features are concerned, almost every card out there is going to play games and homebrew, which is probably what you plan to do with it. Everything after that I'd consider a frill... and it's these frills that separate a $6 card from a $50 one. These may manifest as slightly prettier menus, homebrew integrated into the firmware, folder navigation, special ingame features, savestates, or what have you. These things may be
nice, but as far as I care, they're far from necessary, and I'd probably never use them. When people say sorting and folder navigation... or reskinning on the fly are worth an extra $5... well, starting from a $6 card, that's nearly doubling the price for something so simple. Personally, I'm buying the card to play games, not to stare at the menus (and as far as playing is concenrned, the DSTT is apparently one of the quickest around). The choice is yours, though.
To take this in a slightly different direction, though, I debated which to get between the Acekard and DSTT for a while, but finally settled on the DSTT. The thing that tipped me over the edge is something that people never seem to notice or discuss, but I think it's important to think about--build quality.
http://www.dealextreme.com/details.dx/sku.15613
http://www.dealextreme.com/details.dx/sku.16142
If the pictures on DealExtreme are to be believed, as well as reports I've seen elsewhere, the build quality of the Acekard is severely lacking. Awkward clips with plastic shavings hanging out holding the shell halves together, messy injection points, strange texture, and slightly-off shape really bug me. I don't want to stick something like that in my expensive piece of consumer electronics. I can only picture it grinding or scratching every time it's inserted. The DSTT on the other hand, seems really close to an actual DS card (comparison
here). Note that the Acekard 2i seems to fix this, but I'm not about to pay twice as much (almost four times as much as the DSTT, then) just for acceptable build quality. I don't even have or want a DSi.
I hope this has helped you think more about what you really need. I'm sure someone will disagree with me, but that's kind of the point--everyone has different priorities and needs when looking for a flashcard. You just have to figure out what your own priorities are.