As far as I am aware there have never been any clones of the EZ4. But yeah it is legit, the EZTeam sent us one last year
http://gbatemp.net/threads/new-ez4-sdhc-in-house-at-gbatemp.381182/
Unless you manage to find an old second hand M3 or G6 or EZ3 (note not a EZ 3 in 1) then you are unlikely to find a better GBA flash cart out there, though the EZ4 is by no means bad and you should be able to play just about every game in the GBA library flawlessly.
It is troubled by the same games that trouble all flash carts (give or take RTC for those carts that have it)
http://gbatemp.net/threads/buying-a-gba-flash-cart-in-2013.341203/page-18#post-4756995
Also some games you might want to launch by holding L and pressing A rather than just pressing A (it forces a hard reset which sorts some issues).
You need to patch games to run. The EZTeam provide a program called EZ4client for this, there are various alternatives that people have made in recent times and I suggest every GBA flash cart owner and emulator user has a copy of GBATA from
http://www.no-intro.org/tools.htm to hand.
I am not sure what else there might be to say, I suppose I better cover the NOR/PSRAM stuff though.
GBA games need fast memory to run from, faster than NAND memory as seen in SD, CF and general NAND memory chips. NOR based memory and RAM is fast enough and the EZ4 has one of each.
GBA games other than the shrek films which were only dumped the other month (
http://gbatemp.net/threads/undumped-gba-shrek-videos-analysed.404837/ ) went up to 256 megabit (32 megabytes, the terms will get swapped endlessly in discussions of GBA flash carts so just remember to divide by 8), though most are 128 or less; there is a full list of the 256Mbit titles in buying a flash cart in 2013 post I linked.
The EZ4 aside from the lite compact (which has less) and the lite deluxe (which has more) has
128Mbit of RAM, this is fast to write but loses the data when the power goes (your save is elsewhere and what the battery is responsible for). When you press A (or L and A) in the game selection list this is what you will be using.
256Mbit of NOR, this is slow to write and has another quirk but it is there until you delete it. You go through this menu first when you go to select games and any games written to the NOR will appear there. On the NOR screen start should format the NOR and select will delete the last game on the list (this would be the quirk -- NOR in this arrangement is first in, last out so if you have roms 1,2,3 and 4 and want to delete 2 then 3 and 4 will also be coming off. In the file selection menu then pressing select should give you the option to write a ROM to the NOR. You can write small games if you want a slight speed boost in launching if you like but most prefer to have a large game on there (personally I am not a fan of the offerings from the 256Mbit library but to each their own).