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Are you came here from the past? :O

I recommend you Ez Flash 4 - almost 100% compatibility, no need for convertions.

I've used both flashcarts
How this works: rom is transferred form SD to a ram chip which emulates a proper cart (from GBA point of view). Roms must be patched before they can be used- both carts provide SRAM based saves only while games were also made for FLASH saves.

SuperCard:
-better pc side software, supports batch operations
-ram chips used are fast enough- large number of games suffer from slowdowns (like 1/2 of original speed)
-game saves must be transferred to SD manually (there is no autosave, saves in SRAM are not preserved after powering off)
-cart supports additional features like real-time saves

EZ Flash:
-there are no slowdowns
-gamesaves are copied to SD automatically (but you can prevent from saving)
-much better compatibility than SuperCard
-32MB games must be copied to internal flash memory before playing (~4 minutes, next runs- instant), RAM supports only 16MB
-you must use MiniSD card, using microSD->miniSD adapter is not a reliable solution
-you must use 8.3 names (8 characters for name + 3 characters for extension), cart has problems with longer names
-pc patching software is a pain in the a** to use, no batch support (one rom at a time)
-no realtime saving etc
-SD slot position has no drawbacks, at least you won't loose the card by accident

My personal experience- avoid SuperCard if you want the best gaming experience. EZ Flash drawbaks (proper card, proper names and pc software) are not that bad

and with Ez Flash 4, if you buy NDS you can also use it with them.

Same for SuperCard

Thanks for all the input everyone. Some of it sounds like a bad weekend in vegas, haha. No wonder we don't hear alot about this stuff anymore...
(EDIT: these things are pretty rad.)

I got my Supercard SD(s) today, for personal use on the GBA-SP, Micro and DS Lite and after some troubleshooting, i find that it works quite well for the price (around $25). It plays GB/GC/GBA, NES, SMS, GameGear, PCE (Turbografx-16). That aside, the only ones that have to be patched (with batch support) is the GBA. I subdivided the games by system. The system menu can only handle so many files on the list, which i fixed by subdividing the games alphabetically (#, A, B, C, D, etc...) and now it runs like a dream. I've only had a small handful of rom incompatibility issues, but it's run aprox. 95% of the games that i've wanted to play on it. Despite that, the only other issues i came across was that the NES games need to have the speed settings adjusted, once per NES emu play (i.e. initially, returning after exiting to play other system roms (or powering off) and returning). Not a big deal, you do this by:

- push L + R together (to get to the system menu)
- go to "speed hacks"
- push A (takes you to the only three speed options there)
- turn "PPU" and "JMP" to "ON"
- go to "Autodetect speed hack" and push A
- then just push B twice, which takes you back to gameplay

The other emulators besides the ones above run acceptably or better. The GameBoy Advance emulation is right on the money, i'm assuming because this IS a GBA, and the roms are right at home here...
The roms that run on this don't need to have the buttons reconfigured as they are all (excluding the NDS) two face button controlled systems anyway. Perfect to give to a child for some retro action or a nice and cheap alternative for yourself from other, more expensive or shoddily made, chinese devices...
I bought 4 of them and i don't regret it at all. Each one even came with a free, higher quality Mini-SD card adapter from Kingston.

Does anybody know where to buy an EZ Flash IV? I tried to find a shop but it seems as if the card is sold out everywhere. 

Personally, I stand by the M3 Real in comparison to the Cyclo and EZ Flash IV.  The featureset is significantly improved over the EZ, for instance there is no 8.3 naming restrictions and the M3 has realtime saving too.    Sadly the complete bundle, which comes with the GBA cart that you need for GBA ROM playback, is (was?) fairly pricey.

~edit~

Or not, it seems the Perfect Bundle (M3DSR + GBA expansion cart + rumble pack) is only $35 now.   Bizarrely there's no pack with just the DS Real and GBA, you have to buy the one with rumble too.   Meh.   Oh and it only works on the DS Lite, the original Fat DS isn't compatible with the GBA expansion.