current competition:
1)GPH WIZ*
2)GPH Caanoo*
3)Hard Kernel: Odroid*
4)Nintendo DS (dsi & dsi xl)
5)Sony PSP
6)iPod/iPhone
7)Dingoo*
future competition:
8)Play Station Phone* (Sony Ericson Zeus 1)
9)Panasonic Jungle *?
*-open source handheld
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my opinion of each
1)contols suck, esp the action buttons & screen tearing but probably the best open source alternative I feel currently available
2)I have one, it's fun, my battery life sucks, and the speed/quality of the available emulators are a litttle lack luster, other than that, great device, decent speeds etc.
3)I have one, controls suck, but android brings with it all the same emulators as the pandora and full speed (N64 emulator not available yet but it's actively being worked on) ps1 emulator is in a better state than pandora from what I gather.
4)I have one, with flashcart, if you get a dstwo it has full speed emulation for most older systems gba, snes, gb, gbc, sega, mame etc etc, but no ps1+ emulation. but it has the added benefit of being the only handheld to be able to play NDS games. Depending on your wants, this may be your best option.
5)I have one, used to be king as far as console emulation goes, but it has dropped to the least possible pandora competitor for what it does due to it's speed. It does have the best compatibility of ps1 games I know of due to sony releasing a full speed ps1 emulator for it.
6)I have one, if you hack it, it has most of the same emulators as the pandora, to include N64 emulator, but lets face it, touch screen controls knock this out of the competition. It was mentioned due to the amount of software it has, and it's MID like functions. If you add in the iControlPad then this becomes an option agian.
7)I have one, the stock emulators are good, but the speed is kinda lacking, if you get a minisd (rare/stupid format sd) and put dingux (linux) on it, the speed and compatibility goes way up, due to it's limitations in performance, and the fact you are currently unable to completely replace the OS with dingux it makes gaming on the fly kind of a pain in the ass to start up, and finally the shoulder buttons are bad, but it's real cheap and great bang for buck
8)all the available emulators of android but with decent gaming controls unlike odroid and it has a 3g modem built in. If sony makes a psp emulator for this like they did with the ps1 for the psp this console will be a mainstream king and would make me quit my pandora preorder without even blinking.
9)mmo's in a handheld, and focus around free online play, said to be android based, if this brings some decent mmo's to android or has some other unknown benifit. It's the pandora design I love and gaming controls that don't suck, 3g, and Android which I really wish the pandora had.
EDIT:forgot the dingoo (love that little guy)