The above is personal testing and community feedback, not purchase advice. Please decide based on your own needs and budget.
When I researched flash carts recently, I played GBA ROM hacks. I kept getting crushed when playing Pokemon ROM hacks on Air. I found save states were a lifesaver. Later I got the EZ Flash DE-B, and immediately the experience felt completely different.

Disclaimer
This article refers to the selling price of distributors in China. The actual final price is subject to the distributor.
If you want to ask about game compatibility, please include the specific game version number in your comment for reference. Due to copyright and community rules, I will not share related ROMs, and I hope everyone complies.
Based on my own tests and community feedback, actual performance varies by firmware version and game.

This test uses a DS console.
First, the conclusion:
EZ Flash DE-B is not a replacement for Air. They have completely different positioning. If you play GBA seriously, especially Pokemon ROM hacks, the experience is noticeably better.
$44.70(dealer price at the time of writing, final price subject to actual cost). I hesitated for two days before ordering. Now I think it is worth it — not because the spec sheet looks good, but because I no longer need to fight with the cartridge.
I own three cartridges that support RTC (real-time clock): Chis Flash RTC, EZ Flash Air, DE-B.
Chis Flash is cheap, suitable for DIY users who like soldering. Air has good value for money, great for carrying around and playing during commutes. DE-B costs more, but it shifts your focus from troubleshooting the cartridge to enjoying the game.

I used to have a misunderstanding. I thought the NOR area was just extra storage. It was not until I used DE-B to play a 32MB ROM hack. When I booted it from NOR for the first time, the load speed was as fast as an official cartridge. That’s when I fully understood — NOR is not storage warehouse, it’s a race track. I used to take the bus, now I can drive.

Hardware: 512Mb + 960Mb NOR area, supports ROMs up to 512Mb (previous limit was only 256Mb). Total NOR flash capacity is 960Mb, with roughly 120MB usable space (M here is the common Mb unit for flash carts, converts to around 120MB, enough for several 32MB ROMs to swap between). You can delete any ROM in NOR and flash new ones freely. Press START to check usage status with one click. These two upgrades look minor, but they are really convenient in actual use.

Power consumption: According to community test results (tested under same brightness, same game and same battery state), it uses roughly 30% less power than EZO and SC carts. It draws slightly more power than official cartridges, and a little more than EDX5 Mini. From my personal experience, for a 2-hour round trip commute, DE-B drains battery noticeably slower than Air. No strict instrument tests were done, for reference only.

The new cart and EZO do not require kernel files on the TF card. You can just put games on it and play. However, kernel update files are not interchangeable. It uses FRAM chip for saves. The battery on the cart only powers the real-time clock. Normally, after FRAM saves, the save file will automatically write to the saver file on the memory card on every boot. So saves will never be lost no matter how long the battery lasts. Rumble support works for all NDS games that need rumble expansion pak, plus GBA Drill Dozer. When inserted into NDSL, it sticks out a little bit, same as official GBA cartridges. This is normal. It does not affect insertion and removal. Just pick a slightly loose case; it will not press against the joystick.

GB/GBC/FC games run via emulators, not native hardware compatibility.

Compatibility:
DE-B supports about the same range of games as Air. The differences are mainly in loading method, smoothness and patch support. Some 32MB GBA games loaded from memory card perform nearly the same as Air. Games with heavy text and frequent scene changes like Mother 3 do not feel like waiting for the cart. Demanding games like Tekken Advance can run smoothly directly from the memory card without flashing to NOR. This is a very important feature. Previously everyone assumed large games had to be flashed into NOR. Now DE-B gives you an option: just play directly if you don’t want to flash.

Patch and Chinese support are what I find noticeably better than Air. Patched Chinese ROMs you downloaded can be played directly after loading. For Pokemon ROM hack players, it removes the worry of “will this even run”. My friend and I tested several patches, and DE-B is much more stable.

Mode B is one of the key differences between DE-B and Air. It has three powerful features: GBA & DS game link, Pokemon series data transfer, and rumble support for selected games. Single-cart mode can flash the first ROM on the list into NOR for standalone play.

Pokemon series data transfer: I tested transfers from GBA Gen3 to NDS Gen4. Multiple transfers completed without failure. Still, I recommend saving before transfer. Some hacked ROMs need separate testing for transfer compatibility.

Real experience:
I played a Pokemon ROM hack on Air. Opponent’s team was far higher level. I fought for almost an hour, nearly getting wiped out every time. Air has no save states, so I had to use cheats to modify HP to get through. After switching to DE-B, I pressed L+R+START to make a save state halfway through the battle. I went out for a 30-minute walk, then came back and kept playing, and won on the first try.

Save states are not perfect: some games occasionally have audio glitches after loading a save state. It is recommended to create multiple save state slots and alternate between them. Also, alternate between save states and in-game saves; do not rely only on save states.

It is best to turn off cheats when making save states to avoid compatibility issues.

DE-B can also play official GBA movies. It is not practical today, but it lets new players know this feature really existed back then.

3-cart comparison:
Chis Flash RTC ($14.65, dealer price at time of writing): For hands-on DIY users. You need to solder, flash firmware and configure save solutions yourself. It offers high freedom, but it is not plug-and-play.

Air (great value): Good for casual play and carrying around for commutes. But when you play Pokemon ROM hacks where save states are critical, you will miss DE-B.

DE-B (,$44.70 dealer price at time of writing): The extra cost shifts your effort from troubleshooting the cartridge to enjoying the game. If you play Pokemon ROM hacks seriously, use Chinese translation patches, or even want to try official GBA movies, this is worth the money.

I am not saying DE-B replaces any other cart. They have different positioning. But if you ask which cart to pick for serious GBA play, my answer is clear.

Back to my history of troubleshooting:
Over the years using flash carts, I dealt with lost saves on SC carts, struggled through games using cheats on Air, and now with DE-B I can make a save state, go for a walk and resume playing. The hassle has not gone away, it just moved. Previously I fought with the cartridge. Now I fight with the game. For me, that is enough.

At 44.70 $, I am very satisfied for this price range. Firmware is still being updated, so it will get better over time. I tested with the stock firmware that came with the cart. I suggest checking for the latest firmware after you receive it before testing compatibility, as experience may vary between firmware versions. If you want to confirm compatibility for a specific game, leave a comment with its exact version number and I will test and reply.

Thanks for reading.

Hope your next flash cart lets you spend less time fighting the cartridge and more time fighting in the game.