Summary: I work at H & R Block and I must say that this printer is perfect for our needs (over tens of thousands of pages per year). This printer has performed perfectly for over five years. The only time this printer ever complains is when it's out of paper. This printer is a true workhorse. No, you will not be able to print photographs perfectly. But, yes, this printer WILL perform admirably for tens of thousands of pages of written text documents like tax returns.
Summary: T644 is an good work horse and but is generally more expensive than the T642. For the general user the T642 will do what the T644 will do. Both the T644 and the T642 have a high monthly duty cycle and I would recommend the unit if you already use T642 or the T644. Otherwise I might advise you to consider another vendor, Happy Printing (HP).