I use an EZFlash V and love it. The stock firmware is a bit meh, but update it to the latest version and you'll be hard pressed to find any games that don't work. Even Portrait of Run works peachy.
One big drawback to the EZV is that you have to manually choose the save file settings, though. Thankfully, there's an updated list being maintained that you can just copy onto your cart and not have to worry about it.
Hell, this wiki does a pretty good job of covering it all. That said, I'm looking forward to getting an R4 whenever I get another DS. I'd like to be able to more directly compare the two, plus both are made of equal parts awesome and win.
I do kinda wish I had a slot-2 solution setup so that I could dump the save files from my original carts. One of the huge advantages of playing off the flashcart is being able to back up the saves to my pc (or swap 'em with other people). :(