A neat idea, but a flawed one.
It was, in fact, a two in one system, Sega Genesis and Sega CD as a single convenient unit. I think it was meant as an alternative to the modular design of the whole Genesis suite, i.e. it's a 16-bit cartridge system, add another unit and it's CD-based, add another and it's 32-bit. But what if someone just wants Sega CD games? They don't want to buy an extraneous console. CDX was the answer.
Of course, you still had an overpriced system. And the 32X wasn't included. And there simply wasn't enough consumer interest and hence, not a large production. As an attempt to revitalize the Genesis market, it was a flop.
It is, however, next on my list for my console collection.