I am a ludicrously lucky soul who picked up two copies of DDR Mario Mix with dance pad for $5.25 each at kmart just prior to Christmas! However, I have not seen it anywhere else. toysrus.com use to have it listed, albeit always "out of stock", but it took the listing off recently. It isn't found in-stock anywhere online except on ebay (well, not in the US - the European and Japanese versions still seem to be available in those countries.) I sold the game-only from the second set on ebay, keeping the dance pad from it, and gave the first box set plus the extra dance pad to my kids (5 and 4) for Christmas. It's nice to be paid to give a good Christmas gift.
Alas, it is too hard for them even on easy. Actually, my 5 year old is starting to be able to "complete" some of the easier songs on easy setting and with encouragement I think he'll start doing better, but he doesn't actually want to play the game now because he thinks it's too hard. For myself, though, I really love the game. There are 5 difficultly levels for every song and within those difficulty levels, some songs are easier and some are harder. For a beginner, even the easy level is challenging on some songs.
IMHO there is plenty of challenge for everyone. If you consider only "completing" the story mode to be the goal, then you will finish the game in no time, especially on Easy setting. Actually, in Story mode there only seems to be Easy and Normal difficulty settings, but I suspect beating it at "Normal" will open up harder difficult options (I haven't done so yet, mainly because I've just been playing in free mode.) In part it's easy to beat story mode because you can "complete" a song as far as the game is concerned while getting a "D"! In fact, I'm fairly sure I've seen an "F" and still having it say it was completed, but don't quote me on that. So in that sense it is easy, but if you are trying to do well, getting an A or B on songs, then it is much harder. The "Mush mode" option also adds some additional difficultly.
Myself, I've so far only been able to get an A on songs at "Normal" level (if I recall correctly, the levels are Easy, Normal, Hard, Very Hard, Super Hard), but I should note that this is the first DDR type game I've ever played. Still, I can't imagine that even DDR veterans would consider the faster paced songs at Super Hard level to be easy to get an A on!
Oh, and it is good exercise. Fun exercise, and you can't beat that. For that purpose the difficulty doesn't even matter that much. One thing though regarding this... it has a calorie counting feature, but I can't figure out how to get it to work. I entered my weight, but it just shows up as asterisks and I never see any data collected for me. Perhaps it doesn't like weights that are above little kid level weights. I doubt that the calorie counting is very accurate, but it would be fun, perhaps motivational, to see it anyway.
IMHO it's a great game. I don't know if it's $150 worth of great game, ala ebay, but I'd certainly be willing to part with the full regular retail price for it if you can find it and probably even a bit more. Really, I was thinking just the other day that if the copy we have ever got scratched and became unplayable, I might just pay $150 to be able to continue playing it again. Maybe. I wish I'd kept the second copy of the game as a backup, but it's too late now. More likely, I might install one of those system mods just so I can make a backup copy!
Also of interest to me would be to get the Japanese version of the game (requiring system mod or an Action Replay to play it), so I could get the j-pop songs that I presume would be on it. Oh, and buying the Japanese version and a system mod or Action Replay would be cheaper than buying a US version on ebay. However, of course, it's in Japanese! You could probably figure it out, though. I have read, though, that at least with an action replay, the memory card doesn't work right for japanese games (even if you use a dedicated card) and so you can't permanently unlock anything. If that's true, that would suck and make it a far less appealing idea. I'm definitely not sure about this, though. If someone knows for certain, I'd love to hear about it.