they have been on the market for a few months.
i highly recommend against such a unit as the only ROM drive on the PC.
normal usage of a CD-ROM drive does sequentially decrease the life of the drive. so expect to buy a new one every few years depending on use (can be as much as one drive every year).
now imagine the wear on a DVD drive or a CDRW drive which cost several times a CD-ROM drive itself. and the combo drive which costs even more than an equivalent DVD and CDRW drive (that's 2 drives).
it is great for system integrators who want to save some hassle and charge their customers even more or your casual user... but for people like me, and other power users, these drives are a waste of time. and remember, faster DVD-ROM and CDRW drives come out every so often, so imagine the replacement factor which power users undergo.