Mostly, I play DDR Extreme2, which has an awesome 'Endless' mode. With its very short breaks, and high difficulty, that's a great way to play for exercising (or for getting better at the game, for that matter).
But a while ago, I bought DDR X (it was cheap). I finally tried it today.
Since I wanted to play for a while, with minimal interruptions, I put it into exercise mode (after noting that it doesn't appear to have any equivalent Endless mode). Aside from the one Pet Shop Boys song, the music was pretty lame (I did only look at 25 or so songs, though). And the difficulty? After the first two songs, I put it in its hardest difficulty setting.
On the ten-point scale on which every song is rated (and which on previous versions seemed to be comparable), this was nines, tens, and elevens(!). And yet it was easier than the eights and nines from Extreme2. None of the songs would have killed me if I'd not been in exercise mode; only two would have even been close.
Overall, I'd rate it as LAME. Not sure if I'll play it again in the future.