I have a EZFlash 3-in-1 Expansion Pack with my R4DS and it work fine for me.
I use the GBA play, the RAM for the Browser DS and the the Rumble for some games.

 

If you're in any interest of using a slot-2 with your DS for any reason, be it GBA games, memory expansion, or rumble, the EZ 3 in 1 is the best price/functionality cart out there, and DX have the best fixed prices for EZ 3 in 1 on the market. I ordered mine from DX on 19th last month, yes it's worth buying.

Let me add one thing. It might be possible that after you recieve your card you find it non-working with the available software. In recent batches I want you to know they made a hardware change and placed in a new chip that makes it undetectable by Rudolph's and other popular software. I want to make this known because the EZ 3 in 1 comes with no manual or software and may be frustrating for some users.

If your EZ 3 in 1 is not detected by Rudolph's, download GBAldr.

http://nds.cmamod.com/2008/01/27/gbaldr-3i...raction-update/

This version was released just a week ago and is currently the only software capable of managing these new chip EZ cards(other than the manufacturer's own EZflash slot-1 solution). I believe that if you order it will most likely be of the new batch because I've been seeing many posts about it on their boards lately.

Good luck mate.

 

I've been seeing many reports lately of allegedly "broken" EZ 3 in 1s to people who are using it without the EZ-V slot-1 device. They began this chip changing practice back in december, if I remember well, and for about a month nobody had any idea how to make these new carts work. Personally I prefer Rudolph's for its interface and I'll be waiting until chip support is implemented.