In a moment of shopping madness I bought a MMC replay. I got it yesterday and have been trying it. So far I have to say I'm saving up for a 1541U.
You can't play games that are on multiple disks and multiload games don't always work. On top of that as far as I can tell the Action Replay software side of things is completely separate from the MMC side so that if you load a game via MMC you cannot then use the Replay software to poke it and if you use the Replay you cannot load anything from the MMC. All in all a very limited use product that is highly flawed.